Kill every bearyena you see and you'll eventually unlock them in the mutation menu. I can't remember the exact amount, but I think it was about 30. You have to let the babies grow up before killing them though. Otherwise they won't count. Tamed bearyenas don't count either, so there's no point in wasting food on them.
I just noticed that I have a leech notification, but when I click on it, it just shows an empty tile. Does this mean it went away?