The Plaque Monster

The Plaque Monster is an educational movie seen in "Arthur's Tooth". The titular villain attempts to destroy Toothtown, until he is confronted by the Flighty Fluoride Scour Rangers. They defeated the plaque monster by wrapping him in flossing thread and tossed him down a pit.

As you can see in the stomach part, there are eyes and mouth holes. This means it is probably a person wearing a costume.