Adolf Hitler was an antagonist in the Hey Arnold! animated series and the leader of the Nazi Party. Rather than the actual Hitler, he is merely a fictional version of him that Arthur's grandpa imagined to make himself seem like a war hero.
In Grandpa's story of his days as a soldier during World War II, he recalls working on his military vehicle. When he looks behind him, he sees the Nazi leader Adolf Hitler. They both fought each other in a heated battle. It ended with Grandpa giving Hitler a wedgie and winning the fight. Arnold interrupts the story and tells Grandpa that he didn't fight Hitler and that he made all of this up to which Grandpa admits.