Through the use of developer mode a player can choose to play as Dr. Fetus. He features the special ability of throwing missiles and has the same movement as Meat Boy.
It should be noted that, in the manual that comes with the Ultra Edition of Super Meat Boy, there are diary entries that say "Keith Fetus 8.14.96." It is possible that Keith may be the first name of Dr. Fetus. His Twitter account also shows his name as Keith Fetus.
There is an item in Edmund McMillen's game The Binding Of Isaac called Dr. Fetus that is the called 'Fetus in a Jar'; it makes your character shoot bombs instead of tears, and wear a top hat and monocle. In the DLC Expansion, there is an item called Epic Fetus, which basically makes you the same visually (except for a anger brow) and allows you to hail down missiles.
Dr. Fetus makes an appearance in Super Tofu Boy, PETA's parody of SMB, where he is seen confessing to Bandage Girl and Tofu Boy that he put hormones on Meat Boy so he could act so violently.
As shown in a few comics, Dr. Fetus fiercely hates Meat Boy and kidnaps Bandage Girl to lure him through extremely dangerous paths.
His boss battle first starts with Dr. Fetus trying to hit the player with a homing rocket launcher. The player must run from Dr. Fetus, avoiding the plethora of saws, pits, and Dr. Fetus' missles. Then the player is trapped in a room with Dr. Fetus. There is a switch at the top of the ceiling. The player must dodge the missles shot from Dr. Fetus and hit the button. Then the player must run into another room and the floor will collapse, resulting in Dr. Fetus falling into a pit. When he lands, his glass jar will break, revealing his body completely. As he rolls around in pain and you finally reunite with Bandage Girl, he will press a button, causing "his castle" to start falling apart. Then the player must go all the way back through the level (although with some notable tweaks) with Bandage Girl piggyback riding you (this does NOT affect your movement) and survive to beat the boss. This is the only boss that has a dark world variant.