The Beasthrope form is a beast-like version of the animals they are. Picture a werewolf.
But animals are beasts. So, a beast-like version of a deer is literally just an animal-like deer. Which is a normal deer.
I get the idea, though. In your defence, while I was sitting here trying to think of a good description for what you meant, I was having trouble thinking of something too.
So let's just go with "beast-like".