The "Bus Monster" is an urban legend about a supposed creature living in the ominous and dark back of the bus from the episode "The Bus Boy". The legend has many variations, but the main thing they all have in common is that that the story involves the demise of "Little" Billy Bloomberg at the hands of an unknown menace.
German Boy's Story
According to a candy-loving german boy, Billy was a hefty food lover who would eat candy and drink hot coco on the bus. One day, when the bus hit a bump, Billy spilled his hot coco on the bus driver's lap. In her agony she nearly ran the bus into an ice cream truck. When the danger passed, the angry bus driver sent the boy to the back of the bus were he encountered a giant hungry monster hidden in the shadows which ate Little Billy whole, leaving behind only one bone.
Goth Girl's Story
According to a young goth girl whose mother claimed to have known Billy, Billy was a mean kid who would bully her mother. Once he saw a spider and he crushed it. Before the spider died it called out for help. Thousands of spider then appeared and wrapped Billy up in a web and dragged him off to the back of the bus and presented him to the giant spider queen who then ate the helpless boy.
According to Douglas E. Mordecai III, Billy was a nerdy boy sitting in the back of the bus when suddenly an inter-dimensional time lord opened up a hole in the space-time continuum. The hole then began to pull in everything around it, and though Billy held on to his seat, he was eventually sucked into the hole by a bipolar tachyon vortex which trapped him in a time egg.
The True Story
Billy fell asleep in the back of the bus one day and his chewing gum fell out of his mouth and it stuck his hair to the seat. The more he struggled to get loose, the more the gum would spread, making him even more stuck. In his struggle, his shoe flew off and broke the light (which explains the darkness in the back of the bus). He then spent the next 30 years trapped in the back of the bus and survived by using his long toe nails to rake in food that rolled into the back.