Au Ra physiology and other lore.

[StarDrake]

I would love to know these 'various reasons', because I (personally) fully expect said horde to be actual dragons only.

There are three immediate ones, one of which is a bit meta.

The first is that it simply makes no sense to add the Au Ra to the game if they are not part of the Horde and are not going to make a substantial appearance in Heavensward outside of player models. The other elements that have been previewed all match Heavensward thematically - Dark Knights who will be of either Ishgardian or Dravanian origin (and given that they love to show off DK with this burly Au Ra, I lean toward the latter), Astrologians of seemingly a combined Ishgardian-Sharlayan origin, Machinists because we'll be dealing with airships and cannon and whatnot a lot in Ishgard, bird beastmen who live on the Sea of Clouds, etc. Adding the Au Ra purely for player use and one major NPC makes no sense - we're going to be seeing a lot of them. And the most obvious place for Au Ra to be is in the Horde, in some capacity.

Secondly, and somewhat relatedly and on a half-meta level: the story has been pushing the idea that at least some Au Ra are Dravanian for a while. Yugiri, first of all, seems concerned about her appearance and how Eorzeans will react to it - and yet, despite how easily Alph lets it slide when he brings it up, by itself this makes no sense in context. Ul'dah is a little hostile to the "beast tribes" and Gridania as a city is a bit wary of outsiders in general, but significant parts of Eorzea, especially Limsa, are places where Goblins and Mamool Ja walk the streets alongside Hyurans and Lalafells. Yugiri has a very "normal"-looking body shape and voice (and, on a meta level, we know thanks to the Au Ra reveal and the 2.5 trailer that ultimately she looks far more "human" than the aforementioned Gobbos or Mamools or whatnot and that she's very pretty) so it seems very strange that she'd think the reaction to her appearance would be one of fear, "disquiet" and hostility...

... unless, of course, she had specific reason to think her appearance would cause some upset. Like the fact that her Dravanian cousins have been participating in a war against one of the nations of Eorzea for literally an era, and that being recognized for what she "is" might lead some to suspect her cause is some form of strange Dravanian plot more than anything else. Never mind running into an Ishgardian by accident, as they may well try to kill her on the spot.

Either way, Yugiri's appearance has been deliberately played up as "dangerous", and I don't think that's just the developers being coy about showing the new race. If nothing else, FFXIV's plotting, especially in the patches, has been too tight for that.

Moreover, in 2.4 they very deliberately introduced this idea of Saint Shiva "laying with a dragon", and the flight of fancy this would inevitably lead to. While I don't actually think this will be the origin of the Au Ra, I do think it's part of Ishgardian folklore regarding the origins of the Dravanian Au Ra. And either way, on a meta level the writers wished for us to make this association with Dravania and half-dragons before introducing the Au Ra publicly at all. I really can't see that as coincidence.

On a more general level, as noted, even what we've seen of the Horde to date has been very varied. In addition to "normal" quadrupedal dragons (of several sizes), there are dragonflies, drakes/biasts, aevis-style ground-walking flightless-winged dragons (who seem to serve as the Horde's general infantry), wyverns, two-legged "dragon giants"... with such company, it is not remotely out of question that Au Ra could be part of the Horde. So long as you have an axe to grind with the hominids, you're welcome to help out, seemingly.

Finally, there's one other thing that is purely in the story.

The Akh Afah Amphitheater.

Specifcally, the Akh Afah Amphitheater. It's named in dragon-tongue, and it seems fairly clearly to be a Dravanian structure (it never showed up anywhere on the old Coerthas map), but it's a properly-made amphitheater, with seating, entrances, and a stage, all scaled for human-sized sentient bipeds. What possible use would dragons have for such a structure? (Cue an image of Svara and Maracco doing a comedy routine? :V) What possible use... unless there is a significant element of permanent Dravanian society who are "human"-scale?

Like, say, some Au Ra?

Ultimately it's largely still conjecture, but that's basically all we have at this point. There is, however, a mountain of evidence pointing to the idea that the Au Ra will be part of the Dravanian Horde in Heavensward - probably a significant part.