William 'Billy the Kid' Bonney is a dead murderer/cowboy and his body is in Springfield. He, along with other dead criminals, come out of their graves as zombies and go on a rampage killing people as they have no guns. Homer goes back in time and warns the people not to get rid of their guns, but to shoot the graves, including Billy the Kid's.
In real life, William Bonney was one of Billy the Kid's aliases. His true name was Henry McCarty.
“That is piano！I said Pian-ee”