Slimer's his own, horrible little abomination that Ray made us keep as a pet. If he was human at some point, he's not anymore- no memory of being human, no connection to it- which is more common than you'd think. Lotta ghosts lose any sense of humanity they used to have. And some ghosts were never human in the first place.

Anyway. He's just an awful little spud who eats everything in sight and makes our lives worse.