The beast varies throughout variants but one remains very similar and that is, he is a creature that is trapped between two worlds. He is both an animal but what the characters in the story do not know is that he is also a human being. The actual beast physically changes throughout the different variants. 

Beauty and the Horse[Denmark] - The beast is in the physical form of a Horse.

Beauty and the Beast[De Beaumont] 
The beast is merely a kind soul in the physical form of a beast like creature

The Frog King[Brother's Grimm] - The beast is not the normal ferocious looking character like portrayed in most variant but instead is a slimy and disgusting frog.

The Tigers Bride - in this variant the beast is in the form of a tiger.