I think the speed differences are one of the (major) unique differences that they had from animal to animal and keeping the fast ones fast and the slower ones slow makes it feel more 'real'. I mean, can you imagine a superfast chick? (Then again there's a chick in a powersuit and a giant rabbit but I hope people get what I mean lol)
I was about to ask this same question. I haven't seen any challenges that increase Speed at all... it's really disappointing. That leaves us with what, 6 animals that can become perfect super animals? Horrible.
Well, damn. That really does monopolize things. Though animals like panthers and cheetahs don't have that many restrictions. So...thats like two ultimate animals, considering the night vision, size, and combat capabilities of the panther they would be the best hands-down.
Though Zebras, horses, and ostriches could have the potential to survive the cavemen appearing if they're well bred enough due to plants not going extinct. They can't fit into places the panther can.