"The Slithery" is a mutant moray eel residing in the sewers of Mutant Town in IDW continuity. They are spoken of as a sort of Mutant Town boogieman, said to kidnap mutant children and take them to their lair and keep them in cages, with the rumor that they even might eat them.
The Slithery is brought to the Splinter Clan's attention when a rabbit woman called Laurel approaches them to help her find her missing son, Felix. Lita pops up and tells them about The Slithery, giving them the accounts she had heard about them. Leonardo rebuffed the existence of such a thing, and later snapped at Lita when she got into a row with her peers regarding The Slithery. Lita ran off and encountered a M.O.U.S.E.R., being distracted by it long enough for The Slithery to grab her.