In their fifth year, James, Sirius and Peter became Animagi to help keep Remus in check during his transformations.

Under their influence, he became tame; his body was still a wolf's, but his mind became less so when he was among them.

They transformed into their animal forms every full moon night and would lead wolf Remus out of the shack and roam the grounds with him.  

It's unlikely that any other Hogwarts students ever found out as much about the Hogwarts grounds and Hogsmeade as they did and Sirius, James, Remus and Peter used their knowledge of it to write the Marauders Map and signed it with their nicknames.