Once the delirium of illness has fled, and Fenrir is once more capable of reason and thought, his first clear memory after being dragged from the river is arguing. The boys are not yet men, barely out of boyhood, pushing and shoving at each other. The wolf is no little amount surprised to see that the larger boy restrains himself from full strength with his scrawny little saviour and it draws him in to the conversation.
They’re trying to decide what to call him.
There are what he later learns are the normal sorts of names tossed around: Ace and Fido and Rover among others. His saviour (Steve, the larger boy sighs exasperatedly at one point) vetoes them all, insisting that the dog needed something more dignified. The larger boy (Bucky! Steve shouts the name as he thumps his shoulder, after “Dog” is suggested) finally gives in and asks him “Why don’t you just give the mutt a person’s name?”
Steve calls him Philip.
Bucky never quite gets into the hang of it and he’s just “Mutt” until the boys go off to war without him.
(Sometimes Phil misses being called Mutt. Most of the time he just misses Steve and Bucky.)