Well, the fact it has to be beast like narrows it down a lot, which is good. Try to avoid things that are overused, like legendaries and the Eevee-line, Lucario, etc. Do you have any idea what you want to do history/personality-wise, or how they came to be a gijinka? That could also help to narrow down choices. Something unexpected like Abomasnow, Bulbasaurline, Krokorok, Hippowdon, Pangoro, Malamar, etc could be really fun to play around with.