Remus Lupin had known from a young age that he was different from other children, none of the other children he knew had experienced anything as harrowing as he had done in his short life, and they weren't left with the knowledge that they would never be the same as they had been before he walked through the woods. Remus knew, despite his parents trying to hide the realities from him, that he was now completely different, he wasn't quiet mousy Remus anymore, he was a wild beast, or he turned into one at least every month.
The beast lived in him and stayed inside for most of the month, allowing him to pretend to be the quiet boy he had always been, but then when the full moon rose the beast within would come to the fore and take over. He had never pictured himself howling at the moon before, the idea of howling at or about anything seemed strange to the placid child, but every month that's what he did. The beast would escape him and spend hours braying at the moon and biting and scratching itself.
Remus would wake covered in cuts and bites, the beast once more dormant. The beast was what he always referred to the werewolf he became, giving it no other name, not even in his mind. It wasn't until his friends found out that he had a name to give to the beast. His friends had dubbed him Mr. Moony and he found he loved it. The beast in him now had a name Moony and it made him sound less horrible and more placid like Remus himself. Moony when in the company of the other Marauders when they finally accomplished the change into animals was hardly beastlike at all. He had found friends, found a pack, and they were good for him.
Moony's pack grew and Remus finally felt that he fitted in with others again, even if they weren't like him. However it didn't last long as his pack was decimated by death and betrayal and Remus retreated from the wizarding world, preferring solitude than trying to fit in where he knew he didn't belong. He only really returned to the wizarding world with the lure of pack.
Moony had not been kind to Remus in the time he had been away, the lack of pack having been hard on him. Remus gained some of his pack back only to lose a member and then expand the pack himself with a wife and child. Remus worried about his child and how Moony would react to him and whether or not he had passed on his curse, but he needn't have worried, Moony saw the baby as his cub, pack to protect.
Remus had always thought of Moony as a beast even after he got his name, and he was proven right when his pack was threatened, but it turned out that Remus himself was no slouch. He worked his way across the battlefield taking out as many death eaters as he could manage in the hopes of giving his pack as good a chance of survival as possible, even though it meant his own demise.