Sock puppet

A sock puppet is an anonymous identity assumed on the Internet for the purpose of concealment.

Examples

John Lott, a right-wing economist who writes about gun control and other issues, has adopted the identity of "Mary Rosh" online. As "Rosh," he pretended to be one of his own former students, declaring, "I have to say that he was the best professor I ever had." [1]
Lee Siegel, a The New Republic writer who, under the name sprezzatura, wrote: "I'm a huge fan of Siegel, been reading him since he started writing for TNR almost ten years ago. (Full disclosure: I'm an editor at a magazine in NYC and he's written for me too.) ... There's this awful suck-up named Ezra Klein--his "writing" is sweaty with panting obsequious ambition--who keeps distorting everything Siegel writes--the only way this no-talent can get him." [2]

Richard Morin, "Scholar Invents Fan To Answer His Critics," Washington Post, February 1, 2003.