i dont know for sure, but i think it has to do with the fact that the ponies are better. in the show, most animals are able to have human emotions, and are pretty smart. some like dragons can even talk, but none besides the ponies are civilised, with huge cities and farms. ponies are just more self-seficiant. and ponies seem to be more than willing to use and farm other animals (like cows), and refer to them as bad words (like mules). im not too sure about griffons, i think they are just angry at anyone, i dunno...