A person who uses a fictional character to represent themselves either in real life or on the internet, said character having at least a 50%-to-50% ratio of human-animal physical characteristics. Term is also interchangeable with "Furry."
Steve: Hey Dan, what's that over there?
Dan: Oh, that's just some Beastisapien.
Steve: He seems to be rigorously masturbating for no reason...
Dan: Yep. Definitely a Beastisapien.