Ugmons

Ugmons (ブサモン, Busamon) are an unusual type of Mysterious Beings created from ugly humans.

Overview
Ugmons are formerly ugly humans, who have become monsters due to bullying, or from being turned down from the opposite sex. The feelings of jealousy or inferiority turn into hatred, covering their hearts with darkness dyed with evil. The strongest Ugmon is a dragon-level threat named Fuhrer Ugly. He is able to grow in size and rearrange his body parts. Fuhrer Ugly was later killed by Bang.

They seem to be common knowledge as the kids bullying Tareo mention their existence.