The skum first appeared in second edition in Polyhedron #67 (January 1992). The skum also appeared in the revised Forgotten Realms Campaign Setting (1993), and reprinted in Monstrous Compendium Annual Volume One (1994).
The skum appeared in the third edition Monster Manual (2000), and in the 3.5 revised Monster Manual (2003).
A skum resembles an ugly, black/dark green fish-humanoid, with muscular limbs, hands and feet which are both webbed and clawed, very veiny and tight-packed flesh, a tail, and fins protruding from the head, tail and underarms.
The skum lives in watery grottoes and caves in broods of 2 to 5 individuals or packs of 6 to 15. As its name suggests, it is a rather scummy creature, and could be described almost as an underwater equivalent of an ogre or minotaur. They speak Aquan, and are lawful evil in alignment.