It seems like since the 1990s, almost every prime time animated series premiered as mid-season shows. The Simpsons, The Critic, King of the Hill, Family Guy, Home Movies, The PJs, Futurama, American Dad, and probably Capital Critters, Fish Police, and Family Dog (that one had 5-6 episodes, so I'm pretty sure that one was a mid-season show). Though I think Pinky and the Brain premiered in prime time as a fall series.
I don't know if having prime time animated shows begin in mid season is because of the time and expense invovled with producing an animated series, but that would make it easier on the productions (I've read that Ren and Stimpy's Adult Party Cartoon was trimmed from 13 to 6 episodes for similar reasons). Of course, what I'm saying mostly applies to network prime tiem series. Cable prime time (or later) shows often premier in fall, though many have a half-seasons worth of episodes (same with many saturday mornign shows). it seems like prime time puppet shows also get lumped in mid season, as was the case with The Jim Henson Hour, Dinosaurs, Muppets Tonight, Aliens in the Family, and Greg the Bunny.
Of course, early prime time cartoosn like The Flinstones, The Jetsons, The Bugs Bunny Show, and The Alvin Show all began in the fall.
