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.