I'm glad the link helped
I was reading through this thread again the other day, read you mentioned creatures from Central America and, immediately, I thought about Aztec and Mayan gods! I mean, technically they're not beasts..but some have zoomorphic characteristics, like the serpent with feathers (Quetzalcoatl...I'm pretty sure I mispelled that one..). I'm quite sure all, or most of the gods had a zoomorphic representation, I read somewhere there was one god (I think it was the god of the underworld) which was represented by black coloured Puma with a supernatural roar or something.
So, maybe you can take ideas from the Aztec and Mayan civilization
I haven't looked into the Incas (S. America), but here's a link if you want to take a look at those ones. If I remember correctly there were some interesting deities too.
Good luck! I can't wait to read your story, it sounds so interesting!