Xemnu is a Marvel Comics villains and an enemy towards Hulk, She-Hulk, and the Defenders - though he has origins dating back to pre-superhero comics, being one of Marvel's horror characters from their original "weird science fiction" tales - he is also the first character in Marvel to be called Hulk, a name he lost after a certain green hero came into being and thus claimed the name for himself.
Xemnu made his first appearance on Earth when his star ship crash-landed on Earth after running out of fuel. When revived by the human named Joe Harper, Xemnu revealed that he was a criminal who had been exiled into space for crimes against the rest of the universe.
In other media
Hulk and Agents of Smash
In this incarnation Xemnu is a wrestler travelling in whole wrold seeking worthy opponents. He offers Hulks a wrestle duel and A-Bomb foolishly accepts this.