Here's an obscure one: The Black Loper (currently being rated in the LA thread).

Basically, it's a short story, with a 3.0 monster stat block attached as an afterthought.

Any ideas on developing some kind of backstory or ecology for this hot mess?

Black lopers do not reproduce. They come from some other plane, but which plane and their method of entering the plane is unknown.
Is there a chance they could have originated from the Far Realm?