What was Remus Lupin's nickname in the Marauders?

The story behind the answer

Moony is Remus Lupin’s Marauder nickname because he is a werewolf, and his transformations are triggered by the full moon. His friends—James Potter, Sirius Black, and Peter Pettigrew—gave him the name and became illegal Animagi to support him: Prongs (stag), Padfoot (dog), and Wormtail (rat). Together, the four created the Marauder’s Map at Hogwarts.

To avoid confusion with the options: Padfoot and Prongs are Sirius and James, while Wolfsbane is a potion that helps werewolves manage symptoms—it’s not a nickname. Memory tip: “Moony” = moon = werewolf.