Barty Crouch Jr. is revealed to have used the alias Mad-Eye Moody in Harry Potter and the Goblet of Fire.
Crouch disguises himself as Alastor “Mad-Eye” Moody, the experienced Auror, by using Polyjuice Potion. In that disguise, he teaches Defence Against the Dark Arts at Hogwarts during Harry’s fourth year and secretly manipulates events so Harry enters the Triwizard Tournament.
The real Moody has been imprisoned in a magical trunk while Crouch impersonates him. The deception is exposed near the end of the novel, when Crouch explains that he served Voldemort and helped arrange Harry’s transport to the graveyard.
This plot point is often misremembered as Moody secretly being evil. The actual character is Crouch Jr.; the real Alastor Moody is an Order of the Phoenix member who remains loyal to the fight against Voldemort.