Alright, since I'm one of the very few native French speakers on this board, I'll give a try and help you in your quest for a french moniker.
Old fur is translated to "Vielle Fourrure", but it doesn't sound good for a serious nickname in French. But it could be used to make fun of an old man while remaining respectful to him. "Vieux Loup" (which translates to Old Wolf) is a nickname often bestowed on old tough guys. "Vielle Branche" (which translates to Old Branch) is used to mean Old Friend and "Vielle Peau" (which translates to Old Skin) is often used for an old person as a very mean way to adress them. "Vieux Sage" (which translates to Old Wiseman) is used for old men with a lot of experience to share.