If you want an idea of what the OP is talking about for the character you should look up Double from Skullgirls.
She's an otherwordly shapeshifter who's every move besides her regular stance, walk animation, dash and jumps change into different characters either already in the roster or are side characters in the story. Her normal stance and her weight are huge but her crouch is very low and her dash is more like a fast teleport than a run like other characters but every attack she has changes into other characters in the game, some moves look similar but have different properties while others are moves those characters have never used (she turns into Cerebella to do a R.Mika style buttslam ) This gives her various different specials and normals that completely change her appearance and hitbox. Her level 3 super is her initation a chain combo with a mix of fast teleports to change between characters as she sends the opponent all over the screen
Here's one of the games early trailers for her showcasing how she plays.
Now I like this idea for Beast Boy but as pointed out by Rathalos, the difference between SG and IJ is IJ is a 3D fighter instead of with 2D sprites/hand drawn frames of animation so unlike Double can't use similar animations already existing in the game to make up his moveset and instead would need to have completely seperate character models for every animal he changes into for each move. Double did get some animations made unique for her and some characters who weren't even playable get attacks during some of doubles moves (peacock's allies, Ms Victoria etc). This means he'd probably take ages to make for having so many animal models for his moves but I could still imagine this fighting style work out and be very creative.
