Tamable Rares:
Scale Belly, the blue and purple basilisk (pictured on the right), had a completely different look pre-MoP. He used to be a generic looking yellow basilisk with blue accents. The paint job he sports now is totally unique, making him a pretty desirable pet for low level hunters. I'm sure someone could even come up with a cool transmog to match him (yeah, mine clashes big time I know).
Scale Belly used to be famous for dropping the Chromatic Sword. This sword has a very unique multi-color glow effect. Unfortunately he no longer drops the sword, and it cannot currently be obtained by any means in game. Well, let me take that back, technically you could still find one on the AH from someone who still has it and never equipped it - where in that case I'm guessing it would cost a fortune.
Kurmokk, the red gorilla, is also tamable. Unlike Scale Belly, he does not have a unique model. He is the lowest level tamable gorilla of this color though, and the only red one in Stranglethorn.
Non-Tamable Rares:
If you look closely at Lord Sakrasis, you will notice that he's wearing a Purple Trophy Tabard of the Illidari (well, the top half of one at least). He spawns across the rope bridge in front of the holy spring on the Naga Island.
Kurmokk, the red gorilla, is also tamable. Unlike Scale Belly, he does not have a unique model. He is the lowest level tamable gorilla of this color though, and the only red one in Stranglethorn.
Non-Tamable Rares:
If you look closely at Lord Sakrasis, you will notice that he's wearing a Purple Trophy Tabard of the Illidari (well, the top half of one at least). He spawns across the rope bridge in front of the holy spring on the Naga Island.