The Rat

In spite of its intimidating and scary design, it's still a normal rat. Both Lady and Tramp can intimidate it and, unless it's cornered, the rat never tries to fight back and flees. (Just like a real rat.) During its climactic battle with Tramp, it's clear that the rat has a disadvantage in both size and strength. However, it still manages to fight back several minutes and to be a threat to Jim Junior's life.

Example of:
Villainous Underdog