I know I'm very behind the times with this one, so a little explanation is in order.

I'm doing a little pet project that runs along the lines of 'if MLP was real, how would it work?' with regards to everything from sociology to geology, and have been going through episode by episode making notes on things I find that are either different from or similar to human equivalents. That said, I just got to S6E16 (To Change a Changeling, Thorax's debut) and would like to share my notes! It's probably nothing you haven't seen before, but I'd also like to get some extra thoughts on this, as canon changeling information is scarce and assumptions should always be questioned.

(Apologies in advance for any wonky formatting - this was copy/pasted from my personal Word doc.)

EPISODE 16:

-Twilight wonders if it’s ‘too soon’ to return to the Crystal Empire for a visit

-But last visit was during S6E1/2 – The Crystalling?

-Notes Flurry Heart ‘has grown so much already’

-Noticeably abrupt change from snow to springlike terrain during train ride – atmospheric invisible ‘barrier’?

-Spike disguises himself… poorly, Starlight immediately sees through it

-Note prior instances where this didn’t happen – Starlight is less gullible than Ponyville ponies?

-Of the three ponies discussing the possibility of Twilight/Starlight being changelings, not a one has any idea of a solution – changeling detection is not currently possible for civilians/the Guard (may be possible for alicorns, but require a lot of power?)

-Note that the rest miss the discussion until ‘Everypony RUN,’ at which point they all bolt off in different directions – return of ‘herd mentality’?

-Cadance uses her and Twilight’s dance to prove Twilight is real

-But Chrysalis directly observed this occurrence, and would likely remember it as the thing that Twilight caught onto her from – why wouldn’t she teach an infiltrator this?

-Sunburst states that ‘ever since Flurry Heart’s Crystalling, the Empire is filled with more love than anywhere else in Equestria’

-Can this be measured? Sunburst is usually a practical and empirical researcher…

-Twilight notes that Flurry has ‘gotten so big’

-Hasn’t changed size at all from S6E1/2 model – direct confirmation of pony model type not matching true age!

-Flurry has experienced significant growth!

-Crystal Guard asks Spike for advice on changelings, citing Spike’s prior experiences with them (S2E25/26? Spike wasn’t directly in the fight, though…)

-Spike kicks a rock with a crack in it, manages to shatter it along crack line (force unknown, depends vastly on the type of rock/crack structure/age/etc.)

-Spike falls down a hole disguised by snow, slides down a tunnel before catching himself from falling off a ravine

-Why is this tunnel/ravine here? Flat tundra seen on outside. Possible explanation: Thorax dug out this tunnel (as well as possibly others?) to hide from arctic winds and cold temperatures, as well as Guards), ravine might just be due to ice expansion/contraction from thermal stress

-Spike runs into a disguised Thorax, mistakes it for his own reflection (but no reflective surface is seen!)

-Thorax is able to predict and match Spike’s movements perfectly

-Spike even touches his own claws to Thorax’s disguise’s claws and doesn’t notice the difference – Thorax may have altered texture on the fly to mimic a smooth surface instead of dragon scales?

-Thorax trips on the ice/snow, causing Spike to turn around and spot him – Thorax immediately drops disguise after this (knowing he’s been found out)

-Thorax shows surprising compassion, saving Spike from falling down the ravine

-Thorax notes ‘the ice is pretty slippery’

-Thorax was present during the Canterlot invasion, notes that even then he had no intention of stealing love and wished to share it

-Important to note that this personality, whether secret or not, was allowed to survive in the Hive’s culture!

-Spike’s spikes shown to droop to the side during moments of intense sorrow/pity

-Thorax has an involuntary reaction of growling and sticking out his tongue due to intense hunger

-Note that Thorax and Chrysalis have the same tongue/inner mouth structure in this form! Chrysalis just has more fangs.

-Thorax has never heard of Spike, either from Canterlot or from Crystal Empire

-Likely has not actually encountered/infiltrated CE yet!

-Thorax openly calls Chrysalis ‘an evil queen,’ states he was raised by her

-Crystal Guards laugh at the idea of befriending a changeling

-Shining Armor states ‘There’s no such thing as a nice changeling’

-Racist… but also warranted, given his prior experience?

-Note that while Thorax is transforming, the snow underneath him isn’t melted – ‘flame’ effect has no actual change in temperature associated with it!

-Also note that transformation is extremely rapid and possible even in his current starved state

-Spike’s backstory for ‘Crystal Hoof’ alias is that they met during the Equestria Games (S4E24)

-Note that this counts as ‘going way back’?

-Spike takes ‘Crystal Hoof’ around the city – and nopony seems to remember that anyone unfamiliar could be the changeling??

-Thorax appears engrossed by the city itself, and appears to quickly start enjoying himself via conversation with the Crystal Ponies

-Spike and Thorax have a conversation that would be very audible to the Guards just feet in front of them, but the Guards apparently don’t hear (even when Spike audibly says that ‘Crystal Hoof’ is the changeling)

-Thorax begins to react to close proximity to Flurry Heart, stating ‘there’s so much love around her’ before involuntarily dropping his disguise and growling and hissing due to hunger

-Spike betrays Thorax, refusing to explain that he voluntarily let the changeling inside the Empire’s borders

-Spike finds Thorax at the same place he first found him, where Thorax confronts Spike over his behavior and Spike admits he was a bad friend for not standing up for him

-Thorax disguised as a rock – changelings can transform into inanimate objects! Spike doesn’t appear to notice any difference even when lifting up the rock – weight is also mimicked?

-Thorax saves Spike again from the ravine

-Spike and Thorax somehow manage to evade every Guard on their way to the throne room (all of whom are frantically scouring the palace and possibly the streets in search of a changeling)

-Spike’s song convinces Twilight, who then convinces the others

-Thorax shown to wipe tears from his (compound?) eye – changelings have lachrymatory systems!

-Important note: Thorax refers to his home as ‘the Changeling Kingdom’

-Not necessarily true, as Equestria has also been called a Kingdom (and the Crystal Empire is neither an Empire nor a Kingdom)

-Thorax states his kind doesn’t know how to ‘create love for one another’

EPISODE 16 ENDING NOTES:

It’s the motherlode, boys and girls. This episode comprises the vast majority of what we know about changelings, at least up until the events of Chrysalis’ return.

For starters, the Changeling Kingdom (yes, that’s it’s official name as of now) appears to have a largely aggressive and discontent culture, focusing primarily on taking love from others and using it for food. This is reflected both in the way Thorax is presented (as an outcast of both his own society, and all others) and his experiences in the Nursery Hive (where even newborn changelings were shown to be aggressive towards one another at birth). Changelings have undoubtedly been exposed to the concept of friendship and caring for one another, but are either unable to express it due to lack of cultural tolerance or simply disregard it due to the same.

Next, we will focus on changeling biology. Changelings begin their life as an egg, hanging either from the floor or ceiling via attachment to some sort of root cluster (each egg extends from a limb, which is in turn connected to a large, bulky mass). The purpose of this setup is unknown, and it is equally unknown how the eggs were created – if, like a true insect hive, the queen is responsible for all breeding activities, or if individual changeling drones are capable of reproduction. Changelings have a few similarities to more standard mammalian organisms, having tear ducts (and therefore a lachrymatory system) as well as a mammalian inner mouth setup (though it is unknown how much use this is for a species that eats and digests emotions).

Speaking of which, this episode also gives strong credence to the theory that ‘love’ can be measured both indirectly and empirically – indirectly via Thorax’s reaction to Flurry Heart, the single most-loved citizen of the Crystal Empire, and empirically via Sunburst’s note that the Crystal Empire has more love than anywhere else in Equestria during the time of this episode. While this ‘love’ is never visible during the episode, it is clearly detected in some way by Thorax – possibly via organs unique to changelings. Thorax also shows an involuntary but important quirk of changelings, in that a starved changeling exposed to a high concentration of love may be unable to resist the urge to growl and hiss at it, possibly dropping their disguise in the process.

The act of changeling transformation is purely a magical effect – the flames seen had no effect whatsoever on the snow and ice directly underneath Thorax, where even a brief burst of flame would have melted the snow at least. That said, it is quite possible that Thorax is able to both mimic textures (such as smooth glass or ice) and modify his disguise on the fly (doing so while disguised as Spike to imitate a ‘mirror’ effect). In addition, context clues point strongly to the assumption that Thorax had never once entered the Crystal Empire prior to the events of the episode (even though he’s been in the area since S6E1/2). However, he was able to come up with the disguise of ‘Crystal Hoof’ – a distinctly Crystal pony from looks alone – before entering the Empire’s borders. Finally, Thorax is shown disguised as either a rock or an ice stalagmite when meeting Spike near the ravine for the second time, proving changelings can even disguise as inanimate objects. From all this, we can make a very important deduction: Changelings do not need a target or even a memory of one to form a disguise, and can modify said disguise on the fly for attributes such as texture, and possibly weight and others (Spike didn’t notice any difference upon lifting the rock Thorax was disguised as). Finally, it’s worth noting that unless Thorax performed some very interesting gymnastics throughout this entire episode, the transformation is actually more likely to be more than just visual – that is, it’s not a ‘glamour’ or illusion, but the actual act of transforming into a direct copy. This would open up all sorts of interesting questions – could changelings who don’t know about the internal organ structure of a target still mimic it correctly? Could changelings get details wrong for a disguise they created hastily?

Finally, this episode makes some interesting points about the timeline so far. Namely, that Equestria Games (S4E24, currently placed as occurring about 6 years ago by our timeline), is long enough ago that Spike and ‘Crystal Hoof’ can be stated to ‘go way back’ as friends from meeting there. In addition, Twilight notes that Flurry Heart has grown significantly since her last appearance in S6’s opening, which is both not shown and strongly implies that at least a few months have passed since S6E2 (currently placed as occurring about 2-2.5 years ago by our timeline).

Unfortunately, it’s impossible to determine this episode’s place in time due to both the controlled weather of the Empire and the constant tundra of the Frozen North.

TL;DR:
-Changeling culture is (was?) likely aggressive and antisocial.
-Changelings have some similar biology to mammals, but likely not much. Unclear if they mimic internal structures.
-Changelings can mimic texture, weight, and other non-optical properties of a transformation, and do not need a target to 'replace' - they can come up with completely original disguises and modify them on the fly.
-Changelings might have an involuntary reaction to high concentrations of love in close proximity, forcing them to drop their disguise or otherwise 'out' themselves. This could be used to enemy species' advantage when dealing with them.
-Changelings might have an affinity for burrowing or otherwise forming tunnels/holes in structures. Given their aesthetic, this makes sense.

6758719

do not need a target to 'replace' - they can come up with completely original disguises and modify them on the fly.

From what do you conclude this?

6758816
Mainly the Crystal Hoof disguise and Spike disguise. I'll admit it's not proven, but here's the evidence:

-Thorax is strongly implied to have never entered the Crystal Empire prior to the events of this episode (looking around in wonder at the city, among other things). Therefore, it's pretty safe to assume he's never actually encountered Crystal Ponies face-to-face if this is true.

-Despite the above, 'Crystal Hoof' is a uniquely Crystal Pony disguise (you can tell by the eyes, as well as the tail). It's possible he could have found a random civilian to disguise as, but without the show telling us this, there's no way to say for certain. In addition, if Thorax had found such a citizen to disguise himself as, he'd likely at least have made the effort to learn their name should someone recognize his disguise.

-While disguised as Spike to Spike's face, they touch palms. If Thorax's disguised claw had the texture and presumed malleability of the underside of a dragon claw, Spike should've been able to feel that. The fact that he didn't and still continued to believe that it was a 'mirror' of some kind makes me think Thorax intentionally flattened that part of the palm to resemble a smooth surface. Either that, or Spike has much less feeling in his claws than a human or pony does (possible, given he can survive lava, but that claw would have to be awfully numb for him not to notice a change in surface curvature and texture. Hand-based appendages are unbelievably sensitive to that kind of thing).

-There are no rocks visibly quite like the one Thorax disguises as. While it's possible he could have remembered such a rock, this is never demonstrated.

Possible points against:

-Thorax has been in the area since the events of S6E1/2 (The Crystalling), which is long enough ago for Twilight to remark that Flurry has grown quite a bit since then. It's possible he could have spotted 'Crystal Hoof' from a distance.

-Thorax isn't necessarily a skilled changeling, and may have forgotten basic training (such as learning about one's disguise) and subsequently gotten lucky in that nobody recognized him. Given that he spent time around Spike (who was signing autographs, an event sure to attract every citizen in the Kingdom), this is a little less likely.

-Spike might just have no feeling in his claw digits. This is backed up by him: Surviving direct exposure to lava; being used as a pincushion; breaking rocks and lifting incredibly heavy objects (more an example of his strength, but the latter actually does take a toll on finger joints if done for long enough)

6758821 Good points. Except for the mirror. Even if Spike had sensitive claws, he would still be able to distinguish between moving mirror-like surface and a still one.

Thorax would have been able to simulate moving surface in order to fool anyone with at least some senses.

You may be right about changing according to the imagination, though. There was a scene where the changeling student at Twilight school changes to a sea hippogriff when searching for Yona's backpack in the water if I remember correctly. Then again, she may have seen hippogriffs before.