Frosstbyte

I'd never seen it until Zeraph's post 206, and just adopted it since it's way easier than saying "The entity that is pretending to be Locke and other dead people but really isn't and we haven't seen before today who is probably also the weird guy from the beginning of the episode who wants to kill Jacob."

As to your other question, I cannot recollect any suggestion of there being another "power" on the island that's a direct rival of Jacob before tonight's episode, no.

Lostipedia is calling him "Jacob's enemy" or "Jacob's nemesis." I like Nameless better.