Fergus is currently a drunk floating head. He came into being when TheMuffinMan first became a mutant, being located upon his left shoulder. He is simply a second head created from the improbability of defeating the drive multiple times; however, he did not go away when TheMuffinMan became a robot, kittymorph, joker, or any race afterwords. Also, he would be in a different spot of TMM's body after every drive kill until many kills later when he settled onto the middle of TheMuffinMan's lower back with a noodly appendage of his own to manipulate and drink many drinks with.
After one drive kill, Fergus vanished and was no longer attached to TheMuffinMan. Even another kill and becoming a mutant again did not bring him back, so TheMuffinMan had to go find him. Through a forgotten door in the Common Grounds was the Twilight Zone, a place filled with plotholes. Naturally, Fergus was found in there and, upon his rescue from the absurd place, promised to give up drinking. Obviously, he was drinking in less then a few minutes later, but it was a nice effort.
Fergus would afterwords appear and disappear from varying drive kills, being in a different place entirely when not attached to TheMuffinMan, who slowly but surely disappeared from the island.
A while after TheMuffinMan disappeared, Fergus suddenly popped into existence in the middle of the Common Grounds. However, he was completely floating, for once being on the island without being attached to TheMuffinMan. Happy to discover this, he floated around and joined <DICE>, mostly to raid their drinks, and to finally be able to converse with other neat people on the island while not having to have TheMuffinMan in the conversation, whom Fergus would bicker with constantly and never seem to get along with.
Of course, knowing TMM to be a wet blanket, Fergus expects him to show up eventually and staple him back on, effectively ruining all of his fun.
On the day of the events causing Season 2, Fergus was suddenly attached to TheMuffinMan again. Of course, they had such bad timing that right after they reunited, the island went into a blackout and the season split happened. The 'original' TheMuffinMan and Fergus remained together and fine in Season 1, now wandering the seemingly empty island, but another 'original' Fergus was catapulted onto Season 2 and has no recollection of reconnecting with TheMuffinMan, nor of when he joined GERM, but he doesn't remember a lot of his drunken stupors and he likes it that way. Oh, and he likes it at GERM too, mostly because of how much alcohol Bernard keeps around.
Fine, would some info about him cheer you up? No? Too bad.
On occasion, the improbability is really too much for him and he turns into one completely different. Here is a list of known changes.