At last, a worthy subject!
So, the Asterion we meet over the course of the game has a human dong, but that might not mean that was his default in life. I believe it is, though, given the stipulation the minotaur must retain his basic shape, the pp might be included in that.
But I do wonder if he might give it some thought after accidentally seeing Storm's downstairs configuration.
As for Mythicals as a whole, I doubt there's a rule as Kota and Luke hace animalistic penises while Khenbish and Themba have humanoid ones.