Well, there's this story I have published on another site. It's not pony related, but it IS Metroid-related. It's a Fallout-Metroid crossover called Huntress of the Wastes.

In it, two badass loner mercenaries team up, as the Lone Wanderer/Courier, who goes by the nom de guerre of "Hawk" helps Samus Aran through the Mojave to reclaim her gear and complete her mission. Since they've both been shit on pretty hard throughout life and kill for a living, there's not much hope for their futures as things stand, at the beginning at least.

The story isn't overly combat-heavy, but this post is about the series of battles that mark the end of the first "act".

Now, I've got a few "boss battle"-type enemies coming up after a sub-boss whose fight with Samus I just finished. I've already settled on using a Toho monster (as in the people who created Godzilla) for Samus' battle. After all, she is a Japanese-designed character.

For our favorite transhuman, intergalactic, Space Pirate killin', upgrade-tastic femal mercenary, I need some help choosing a Toho monster. I need the creature to be at least 25 feet/8 meters tall, but not more than 125 feet/40 meters tall and be killable by a single badass augmented human with an arsenal of weapons.

If you can help me pick one for her to battle, I'd appreciate it a lot.

For our Vault-born, Wasteland-worn protagonist, I need help choosing a movie monster of American origin. As with Samus, I need the creature to be at least 25 feet/8 meters tall, but not more than 125 feet/40 meters tall and be killable by a single badass augmented human with an arsenal of weapons. Pre-80s monsters are preferred, but if you can find any American movie monster that could fit in the Fallout world, I'd appreciate it. Since there are a lot of references to movies that aren't 40s-50s-60s, I'll take monsters from more recent American movies, as long as they're silly enough. Clover isn't viable because the film is too serious, and the monster's too big. If it's more like a C.H.U.D., or a Graboid from Tremors in spirit, or something a bit nutty like those monsters, it's good.

Any questions are welcome.

In the way of 80's movie monsters of that size, the only things that come to mind are the ants from 'Them' and the original 'King Kong', other than that, the generic kracken comes to mind.

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King Kong wouldn't work, because I really can't see him in a post-apocalyptic desert, and a Kraken even more so.

The ants from Them! are already a large influence in Fallout, where the smallest ants are the size of housecats, and the normal ones are the size of a Great Dane. The queens are the size of a semi truck, but those are as big as they get. Fire ants literally breathe fire, too.

Okay, so major dessert dwellers, hmm... giant king cobra, perhaps?

GODZILLAAAAAAAA

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Nah, it's gotta be something more than just a giant version of an animal. Besides, cobras aren't native to the Western Hemisphere, nor are they as common in zoos or pet stores as the Emperor Scorpion, which is already present in the form of Radscorpions (seriously, the biggest ones are the size of a truck).

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Too big, too iconic, too hard for Samus or Hawk to kill, wouldn't be a good boss fight.

What about a medium dragon or a giant rattlesnake?

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Nah man, the key phrase here is movie monster. It's gotta be something that they could beat, but impressive and unique enough that it's not just killing an up-sized enemy.

I mean shit, I'd have Samus fight Rodan, one of Godzilla's frenemies, but he looks too similar to Ridley.

Would a demon work because I read a book called fable haven and had a demon named Bahumant and is a medium sized like dragon with 4 heads and black scales

You're kinda limiting yourself to what you want to have them fight, I mean, just because Fallout has a bit of an 80's theme doesn't exclude monsters of other time periods, plus, other than Godzilla and his adversaries, what other giant movie monsters are there?

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I wrote that it was pre-80s movie monsters I was looking for. There aren't many giant monsters in movies by the 80s that weren't just generic mythical creatures. I was asking for something that's supposed to be impressive, but is really kind of silly, like you'd see in Mystery Science Theater 3000.

I found a list on of various movie monsters, though most of them are either
1. Giant versions of normal things
2. Godzilla & Co. (Many of which are already Metroid-esk looking)
3. Prehistoric animals
Or 4.Not even giant monsters.
Sorry if this doesn't help you, but it's what I see you're trying to work with.