Well I would imagine that we can either go with the simpler option of the beast looking totally like a normal earth based animal, but with a human/ mostly human like form it can take which would be part of his magic, but other than that a base of magical skills running along the more nature sphere and/or concealment magics. 

OR

We can go with a more fantasy like world with non normal earth looking animals/beasts that all humans think are feral/unintelligent that are considered either monster/wild animals giving us a broader range of options for your beast. Also I would consider these "Monsers" tamable to a degree so there would be wild dog like beasts and tamed ones, but not the. . . . .As in there is no Canis Domesticus, simple a wild dog like thing that has been trained as a pet. If that makes any sense. I always loved the idea of a special kind of people who can tame wild beasts as a vocation. No that the tame animals don't breed, but more that this is way before the true domestication of animals. . . . Anyways, this would be the same as above, maybe with a few more combat based magics/skills(Still with a human/mostly human like form), but this is mostly to allow for us to come up with some funky beasts AS we play :P

Basically it's the difference between Harvest Moon and Rune Factory, if that makes any sense :P Minus the gardening monster bit. Either work for me. One is more simple and less likely to cause conflict, and one's more my area of "Study" as it were.