Bull-E was created when Timmy wished for a comic book world in the episode "The Big Superhero Wish!". Although he was easily defeated by Timmy and his superpowered friends at first, because good always triumphs over evil in the comic world, Bull-E eventually gained the upper-hand when Timmy was tricked into wishing everyone but the villain's powers.
Bull-E is just that, a human bull. He's immune to Profeser AJ's mind reading powers because of his tiny brain. He also has a slight speech impediment (Example: "TINY TURNER MAKES BULL-E MAD!") and attacks by running into his enemies.
According to Bull-E, the E in his name is there because it is "the second letter in death, and the third in die". Bull-E was originally Francis, who was wished into his form inadvertently by Timmy when he wished for the entire world to be like a comic book, thus creating worthy and formidable villians to balance out the heroes that were created. Because of his predictable way of fighting, he was easily defeated by Timmy and the other superhero kids. However, he was handed over to the Crimson Chin who was really Nega Chin in disguise, and the supervillain recruited Bull-E and Timmy's other enemies to fight against them. Timmy eventually discovered Nega Chin's plan, but it was too late, and as he tried to unwish everything he accidentally left the villains with their superpowers.
Bull-E ruled the world alongside Nega Chin and the other villains, although he was ironically the whipping boy of the group. Eventually, the real Crimson Chin and the other de-powered heroes managed to defeat him and the other villains, Bull-E in particular was dispatched by a janitor who wet the floor with his mop, causing Bull-E to slip on the wet floor and fall out of the window of Nega Chin's Lair (with the help of Elmer and his window opening powers). He turned back to normal when Timmy got his godparents free and unwished everything.