Majin Buu - Dragon Ball Z - Image Courtesy of FUNimation
A big pink guy, Majin Buu was created by an evil sorcerer, Bibi-Dee and then put into a dormant state after Bibi-Dee discovered he couldn't control him. Baba-Dee (the son), resurrects Majin Buu and releases him on Earth. Hercule (aka Mr. Satan) convinces Buu that destruction is bad and Buu is actually about to turn over a new leaf. Until, that is, some idiots kill Buu's dog, sending Buu into a rage. Evil Buu separates from Good Buu and the battle ensues. Evil Buu wins, eating Good Buu and transforming into Super Buu. After destroying pretty much every earthling, Buu seeks out Gotenks and ultimately absorbs him as well. Buu is finally killed by Goku.