When Oda follows the northern mythology, then thats lokis favourite weapon, but sure, he doesnt need one.
About Fenrir... Fenrir is the son of Loki. Its plausible he exists. Back then, i described Eustass Kid as Tyr (made a bigger theorie out of it, i can link it if you are interested) and expected Killer to be Fenrir, cause i thought Kid gave his arm to save or help Killer. But it could be also otherwise.