The Last Synapsid — Timothy Mason

Faith, Colorado, doesn't get many visitors. But this spring, a mysterious creature is lurking on the mountain. Fiercer than a mountain lion, it's been hunting pets and horses, and leaving their remains scattered over the mountainside. But what is it, and what does it want? Only 12-year-old Rob and his best friend, 13-year-old Phoebe, are brave enough to investigate.
What they find on the mountain is beyond their wildest imagination. The last synpapsid -- a squat, drooly creature that looks like a dinosaur crossed with a wienerdog not only speaks English, but also claims to need Rob and Phoebe's help. He's looking for his cohort -- a violent carnivore called a gorgonopsid. If he doesn't bring it back to his time, history will re-align, humans will never evolve, and Rob and Phoebe will end up as nothing more than characters in an elderly synpapsid's dream.