What exactly is a Flerken in Captain Marvel? How does it swallow things much bigger than itself? Where do they originate from? Do they show up anywhere else in the MCU?
What exactly is a Flerken?
Flerken is an alien creature that looks like a household cat on Earth.
Flerkens are living gateways to pocket dimensions which are located inside of them. They can keep things like eggs and fanged tentacles inside their bodies, where the latter can be used as an offensive weapon.
How does it swallow things much bigger than itself? Where do they originate from?
This creature seems to have pocket dimensions allowing it to swallow things bigger than the cat.
From Nerdist with emphasis of mine,
a Flerken’s body also possesses “pocket realities,” which are bubbles of space and time that exist in other worlds. They can actually store entire universes in their mouths, and they can travel interdimensionally. It’s not easy (or clean), but it’s possible.
Do they show up anywhere else in the MCU?
It's not clear in the movies yet. However, it seems that Flerken can store an entire universe and regurgitate whenever they want as Goose regurgitates the tessaract in an end-credit scene.