Lil' Doomsday is an alternate version of Doomsday from a paralell Earth in the Multiverse. He looks much like our version of Doomsday, and has similar abilities, but is much smaller and has a considerably nicer origin. He appears in certain issues of Superman/Batman.
He was part of the group of villains brought over from that earth by Mxyzptlk. Unlike our Doomsday, he works with other villains, allowing himself to be dragged around on a chain by Lex Luthor.