Best bet if you're looking for something unique is to check out Petopia.
http://petopia.brashendeavors.net/
Personally I loved my Humar, that black lion from Winterspring. I also had an RFK boar for a while, and Broken Tooth before he was 'fixed' (normalised). Had a turtle for a while, amazingly tough tank pet but couldnt keep aggro, same with the polar bear I had from Winterspring. The problem with tank pets is that while they have a taunt like all pets afaik, they can't
hold aggro like a dps pet can because their dps is lower, and I found myself being hit more than when i was grinding with my cat, so my xp gain was, overall, actually slower since i had to feign/disengage/trap/kite more often, and of course overall dps was lower.
Boars are awesome levelling buddies I hear, because of their charge which deals a high amount of initial threat, and they can be trained Gore now. Another option is a scorpid, although they're more of a pvp pet(high armour and stacking poison that messes with opponents' cleanes) than a pure dps pet. You could also look at getting a bat or owl, they're high dps and have a nice Screech debuff thats awesome for raiding.