Do animators count too?
Anyway, first, we have Doug TenNapel, artist. He worked for a lot of cool stuff, like Earthworm Jim, G.E.A.R. (which I'm yet to have read, if I could only get a hold of it), and MDK, but he is also a fundamentalist Christian. They're always bad people.
And for animators, John Kricfalusi, most famous for the Ren & Stimpy Show. Although he's more so an opinionated asshole than a straight-up asshole, he frequently missed deadlines and got the boot from Nickelodeon. Of course, the later episodes (without his involvement) were finished and produced by Games Animations (and the quality of the show also started falling from there). The less we talk about the abysmal Adult Party episodes, the better.