Fenrir

[Velox]

but the Amdapori summoned Diabolos and they were White Mages the whole group.

According to in-game lore, it was the enemies of the Ampadori (most likely the Belah'dians but was never confirmed) that summoned Diabolos to destroy the White Mages. The White Mages were the ones who were able to re-seal him away deep within the Tower of White, and it's only after centuries of weakening did the seal break and we (adventurers) were called to reseal him.

As for Fenrir, if he had some significant story in FFXIV I'm sure they would have included it. The fact that they didn't simply means, to me, that he's probably just the most powerful of his kind and that's it. Or he's simply a trained "guard dog." Snowcloak's focus was on Iceheart, not Fenrir.