Bart's guilt

Bart's guilt is a manifestation of Bart's guilt.

History
After Bart threw away Lisa's hat, which she loved, Bart started to imagine his guilt making him feel bad about it. As Bart keeps refusing to feel bad about what he did, his guilt kept getting bigger and more gross looking.

It kept following Bart around talking to him, but Bart kept ignoring it, but was getting worked up by it bothering him. It got so bad that Bart tried to give Lisa gifts to get her to forgive him. Bart's guilt then followed Bart to the junk yard to get Lisa's hat back.

Eventually, Bart and Lisa made up and Bart's guilt left the planet until it hits a plane and falls into the cooling towers of the Springfield Nuclear Power Plant and turns into a monster.

Trivia

 * One of its forms looks like Hugo Simpson.