Fenrir

[WyrahFhurrst]

Well, I don't know about Siren there, but the Amdapori summoned Diabolos and they were White Mages the whole group. (I suspect his summoning was to be used as a weapon in the War of the Magi, but he was stronger than the Amdapori expected and wiped them out.) Fenrir, we don't know at the moment. I suspect we will get more with the whole Fenrir mount being a thing in 2.51, and the Mount Guide being a thing.

[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.

[Anonymoose]

(most likely the Belah'dians but was never confirmed)

Proto-Belah'dians. Belah'dia was founded in the Sixth Astral Era, whereas Amdapor fell in the Sixth Umbral after the War of the Magi in the Fifth Astral. Surviving mages founded Belah'diah, and some of the ruins they built upon were already there from the previous Era (as Qarn tells us). These surviving mages are the ones that likely came from the groups using Black Magic.