in the comics he was a man named George Tarleton, who was experimented on by AIM, an arms dealing organization. They mutagenically increased his intelligence, but to the consequence that his head grew freakishly large to store it all, and his body was left stunted. So he was built a hoverchair to get around in that also keeps his body functioning right. He killed the ones who experimented on him, renaming himself from Modoc (Mechanized Organism Designed Only for Computing) to Modok (Mechanized Organism Designed Only for Killing)

In the film basically he was the guy they defeated in the first movie by sending him to the quantum realm, he survived, and it left him deformed with that freakishly large head