It's some sort of Xenomorphic creature.
It devours other creatures and steals their traits, making itself ever more deadly.
I don't have a name for it yet, but something will come up.

They tend to swallow prey whole as soon as their big enough to, but they begin life so tiny and adorable looking that you'd never guess otherwise.

It might have a huge gut after eating someone, but it doesn't slow it down one bit - it's instinct driven brain auto-calculates for the new weight and center of gravity, allowing it to be just as intimidatingly graceful and quick because of it's odd slim musculature.

It's deadliness in a small, vicious package.