Elmo Monster is an insane man and lunatic interviewed by Mario in Mario Interview: "Elmo". He later reappeared years later as a red monster plush of Bowser Junior, unlike Thomas, Elmo Monster did not speak, but did laugh at pretty much anything.
Elmo Monster was one of the burglars in Home Alone 2. He laughed at the rat trap trick, the brick trick, and the sniper rifle trick, but he was shot by it by Bowser Junior. He was the tertiary antagonist of Home Alone 2.
In The Pizza Delivery!, he was accused by Big Bird for eating all the cookies (when Cookie Monster really ate the cookies) and was beaten up by the cops for laughing instead of telling the truth.