Lopsloss only one is known to exist, in the dungeon of Castle Vaunting [I p. 130].

The executioner took a torch from a wall bracket and tossed it to the pit. They glimpsed a mountainous gelatinous mass disfigured by warts, craters, ridges. The torch splashed into water and went down. Darkness shifted: sucking, squelching.
"Lopsloss," said the executioner.

-Book 1, The Wizards and the Warriors (Wizard War)

The Lopsloss is large, ugly and disfigured, its skin covered in warts and bumps. The Lopsloss is a large and ugly creature that lives in darkness and is hyper-aggressive. It is resistant to all forms of magic. The Lopsloss guards the only non-magical entrance into the Tower of Seth.