One last clue first

Final hint: It has something to do with a body part

Eh spit it out

Yeah this is hard

Answer: each monster's rare has more eyes then the common form

Are you serious

Oh my gob. OH MY GOB. HOW DID WE NOT GET THAT.

...OHHHHHHHHH-

...

I thought it was all monsters had an aux seasonal costume

I don't even think Nebulob or Dandidoo have one

^They don't, you might've been thinking of teleport costumes, which they both have.