From what we've seen in Canterlot Wedding, Changelings are able to change their appearance to all pony races-earth pony, pegasus, and unicorn-even the princesses themselves. That's all canon right there.

Now the question I have is can they change their appearance to any other species? Zebras would probably be no problem as they resemble the regular equines quite well, but what about species like gryphons, buffalos, yaks, maybe even minotaurs if Iron Will isn't the only one we know in Equestria? Would they need a certain amount of magic to pull those appearances off, especially the bigger ones? Could they only do ponies and the like? I'd like to hear what you guys think.

Yes she can change into anything more or less her size. She can go anthro if she has some model to go by.

4931840 We don't know.

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Really, it just comes down to your own thoughts and wishes when it comes to Changelings and their abilities. It differs greatly between one author to the next, which I think is supposed to be the appeal of Changeling characters. You don't know what to expect until they're exposed.

The answers to your own questions will help you form your headcanon on Changelings.

The Theory I go by for it is yes, they can change into pretty much any living creature, but depending on the size it can take a tremendous amount of energy/magic/love to do so. Ponies and other equine are easy as they don't change too much. As for things of a grander scale like a goldfish or a dragon, only the most powerful changelings can do so, and even if they haven't stored up enough energy/love to do so, it can kill them.

4931840 What if there's different subspecies of changeling, that mimic the other major species?

4931840 Depends. I think you can for Zebras and pony-looking species. As for the rest go by whatever you want.

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I've also encountered these ideas:
• Changelings only being capable of changing into magical creatures. As that they could sniff the aura of a pony, or a manticore, and could then duplicate that individual. These changelings could not copy dogs or humans, though.
• Several changelings needing to merge together in order to change into something larger, like a gryphon or a Buffalo.
• Changelings being hollow, so one could hide inside another.

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What we know for sure about changeling transformation is very limited. We know that they can assume pony form. They, or at least the queen, can change size and presumably weight (Chrysalis appears to be a tiny bit taller than Celestia, but was able to take the form of Cadance, who is slightly smaller than Luna, and even Applejack). It also appears that they can create fake articles of clothing (Applejack's hat and hair-ties when mimicking her, for instance), and it's possible that certain worn items can be incorporated or hidden with a change (In their first appearance, an armored changeling's breastplate and helmet disappear on transforming, though he's also one of the changelings mimicking Applejack... assuming that armor wasn't created through transformation magic to begin with). Zecora wearing the same markings as everypony else in one of the alternate timelines at least suggests that changelings might be able to disguise as zebras, but that's pretty speculative. And that's pretty much all we know.

As for delving into headcanon territory: considering how big of a size-change Chrysalis is able to do, I personally like to imagine changelings' transformation ability to be very flexible. I figure the main limitation to be personal skill, and that the more extreme the change, the more energy it takes. So, a changeling could presumably turn into a full-sized dragon, but the amount of energy it takes would be astronomical. Similarly, a common changeling could turn into a regular pony, or a big, ruggedly built pony in perfect physical condition, but the latter would take more energy. Considering that I also assume consumed love to be what powers their magic (Due to it's effects on Chrysalis's power) and that the talk of food may mean they need it to survive, that can be a pretty big limitation if a disguise is going to be draining a lot of your life-sustaining energy.

I also like the idea of small, subtle changes. Stuff like muscle tone and bone density, for instance. Small things that might not be noticeable, but can improve a form beyond what it might normally be, with a corresponding increase in the energy it costs.