Tartar Sauce is a Pet summoning item obtained from Iron Crates, which are found by Fishing in any Biome. When used, it summons a Mini Minotaur, which, like other pets, follows the player around and does not deal or take damage. Its yellow eyes glow in the dark, but it produces no other light. When the player gets too far away from it, it will twirl its hammer and "helicopter" to them.
This is a reference to Tobuscus' song, Mini Minotaur, in which it is said that one needs tartar sauce to kill a Mini Minotaur.
Its appearance, the red loincloth and the hammer, is probably a reference to the Maulotaur from Heretic and especially Hexen, where the maulotaur can be summoned by the player to aid them in combat.
In the 3DS version, instead of saying Mini Minotaur, it says BUFFNAME (136) and for the tooltip it says BUFFTIP(136)