Major Mucus is a phlegm-based alien who is the military dictator of "a distant phlegm planet."
In the first video game, Major Mucus desired Earthworm Jim's super suit, which led to the two having a bungee-jumping duel over a pit of snot in Snot a Problem. The Major's pet "Mucus Phlegm Brain" would eat Jim if he fell too close. If the level is completed, the Major's snot wire will split, causing him to fall.
It was during this mission that Jim would acquire a new ally from the Phlegm Planet - Snott, apparently a defector from Major Mucus' ranks. Snott would later help Jim defeat Queen Slug-for-a-Butt in the last level, letting Jim stand on him as he hovered around the Queen.
In Earthworm Jim 2, Jim returned to the Phlegm Planet on his Pocket Rocket to destroy the Major once and for all. Major Mucus used his massive army of things such as exploding sumo-wrestler launching catapults to try and destroy Jim on his Pocket Rocket in the level The Flyin' King.
Although apparently destroyed in Earthworm Jim 2, Major Mucus would reappear as a major villain in the comic book mini-series, after he answers the call of Evil the Cat in Heck during the opening panels of the comic.
Major Mucus is the only villain from both of the original Earthworm Jim games to not appear in the television cartoon series, though a miniature version of him was included with the Psy-Crow action figure in the Playmates Toys toyline largely based on the series. An animated scene featuring Major Mucus was, however, seen in a 1995 pitch reel for the series (and subsequently used in early Kids WB promos for the show).