John Monroe was a lonely student at Stanford University in the late 1960's. His contemporaries rarely, if ever, noticed him and if they did, they would refer to him in derogotary terms, using words like "Weasel" to describe him.
Embittered by this experience, Monroe became a murderer decades later. By this time, he was a teacher at Vandemeer University in Pittsburgh, where a number of his fellow students from Stanford held prominent positions; Arnold Lintel, head of the Vandemeer's engineering dept; Linda Walters, an instructor in music; and physics professor Peter Banks were among those who had taunted Monroe years earlier.
To rationalise killing these three, Monroe convinced himself that they represented threats to his getting tenure at Vandemeer. Monroe devised the costumed identity of the Weasel and brutally murdered Lintel, Walters and Banks, as well as night watchman Chuck Gherkin. Soon afterwards, Dr. Martin Stein, the physicist who is part of the composite being known as Firestorm, arrived at Vandemeer to fill a position of physics professor. The Weasel made two attempts on Stein's life, until Firestorm eventually unmasked Monroe and sent him to prison.
The Weasel went on one mission with The Suicide Squad. They crossed paths with The Doom Patrol in Russia. Mr. 104 attacked Robot Man, starting a battle. The Weasel slew The Thinker when he attempted to restrain him. Rick Flag wore The Thinker's cap and killed the Weasel, The Thinker's last thought.
The Weasel would later meet his demise and has been identified as one of the deceased entombed below the Hall of Justice
Robin: To Kill a Bird
One of Penguins Henchmen is named Weasel, it is most likely not John Monroe, but most likely a thug with the nick name.
DC Extended Universe
Weasel appeared in The Suicide Squad (2021), played by Sean Gunn. He is a member of one of the two teams. He supposedly died while drowning but the very end revealed that he survived.