For cows, I nominate the Evil Cow from Earth Worm Jim... The one Jim's evil twin makes with the evil twin-duplicator machine.
Good cow would of course be the good twin (eg. Original falling cow)
As for Rabbits. I have a candidate, and mark my words! That's no ordinary rabbit. That's the most fierce, vile and foul tempered lil' rodent ye ever set yer eyes upon. It has a streak a mile wide... And huge... Sharp... Teeth! Look at the bones... The bones!
I am of course talking about the killer rabbit from Monty Pythons "Search for The Holy Grail".
As for good rabbits... there are no such things as a good rabbit... Rabbits are evil... End of arguement.
Evil penguins? Mummon almost hit the mark right on the spot. He mentioned the SECOND evilest... The evilest (can you even say that?) is of course...
As for good penguins, I nominate the penguin from Batman... He was such a cuddly happy fella'... No, seriously... You obviously never met my old Latin teacher...
Tm