Fan Nickname: As the game doesn't give you the names of normal enemies until the credits, players tend to give them these. Anyone who has watched Northernlion enough tends to use "human popcorn", "silkworms" and "zambies", in place of their real names (Mulligan/Host, Charger/Spitter, Knight).
Meatboy Lvl. 5—That's what some players call the severed body "generated" by The Shears. The reason behind it is because it used to be possible to have it constantly following you by acquiring five Meat Cubes via glitch exploitation.note It's still possible to acquire more than 4 Meat Cubes, but now the fifth one you get transforms into a new rotating Meat Cube instead of upgrading your 4th Lvl. Meatboy.
An even more common one is "Blue Baby" for the unwieldy ???. For the True Final Boss as well as the character.
Pooters are known as "Grandfather Flies" because their white abdomens look like beards.
Promoted Fanboy: Spider, the mind behind the SpiderMod program (a modified version of Isaac that runs in a separate .exe, is more optimized, and adds a number of features including various cheats and controller support) was hired to do coding work for the remake.
Sleeper Hit: Word of God has stated he never expected Isaac to be so popular. At one point he considered giving it away because he didn't believe anyone would actually want to spend money on something so weird and dark.