Sephirot, Nidhogg, Sophia Mount from Totem Exchange when?

[Nezerius]

I wouldn't expect them to release before 3.55. Nidhogg is only a patch old and still more than capable of wiping groups even at ilvl 250. As for why they're holding them back, they want people to keep farming. Only Sephirot is really "old" at this point. I constantly see Sophia parties in PF and even the occasional Nidhogg. Increasing the bird drops means once people get them, they'll likely have no reason to touch any of them again.

3.55 would be way too late to add the Nidhogg bird to the vendor.

I'd expect them to be adding the Sophia and Zurvan birds to the totem exchange vendor in either 3.55 or 3.56 (2.x primal mounts droprates were buffed in 2.56), as a further incentive for players to continue playing the game. You have to keep in mind that the entirety of the 3.5 patch (3.55 included) will be around for longer than the other patches. Going by the current patch cycle, we'll likely see 3.5 mid or late january, followed by a space of almost 5 months until the expansion's early summer release.