Sockpuppet

From Superdickery

A forum account created specifically to parody, mock or otherwise emulate some event, situation or person. See accounts such as Deadshot, Stephen Colbert and Booster Gold.

Alternatively, a sockpuppet is a secondary account of a forum regular, used for mocking, commenting or simply personal amusement. Notable sockpuppets of this kind include MODOC, Minnie Presidette and Dragon-Wolfe Dolphinn.

Fun fact: Forum regular TSI started out as a 1-joke sockpuppet, emulating the Monty Python "Spanish Inquisition" sketch. By this point, no matter what he changed his name to we would continue referring to him as TSI.