A person with excess body hair wearing only underwear usually resembling Michael J Fox from Teen Wolf. It is assumed that the Under-werewolf possesses special powers so they are generally feared.
"Dude, did you see the Under-werewolf in the locker room at the gym?" asked Bob.
"Yeah, I was scared for my life...I got out of there quick because I didn't feel like getting my face ripped off today." replied a trembling Steve.