Lycanthrope (8)
I believe the answer is:
werewolf
'lycanthrope' is the definition.
(synonyms)
This is all the clue.
(Other definitions for werewolf that I've seen before include "Scarily unpredictable type" , "Person who becomes a monster when the moon is full" , "It's fabulous" , "Lycanthrope, shape changer" , "man going wild occasionally" .)