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.
According to Otto, a food-loving German-accented boy, Billy was a hefty food lover who would eat candy and drink hot cocoa on the bus. One day, when the bus hit a bump, Billy spilled his hot cocoa 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 gobbled up Little Billy, leaving behind only one bone.
According to a Vicky, a girl whose mother claimed to have known Billy, Billy was a mean kid who would bully her. Once. he saw a spider crawling on the school bus window and crushed it with his thumb. Before the spider died, it called out for help, and thousands of spiders came to wrap Billy up in several webs and dragged him to the back of the bus. There, he came face-to-face with a spider queen who desiccated 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, sticking his hair to the seat. He tried to get loose, but the gum spread, making him unable to get free. In his struggle, Billy's shoe flew off and broke the light, leaving the back of the bus shrouded in darkness. 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.