Theory: Aatrox was a horned owl.

As far as I can remember, we only have two sentences from the lore about his appearance, one from his biography that states, "his wings were the golden light of dawn," and one from his short champion description that states his current form is, "a brutal approximation of his previous form." And other than that, we've got his actual model.

So I got to thinkin', "what animal can have gold wings, and also has a face similar to Aatrox's current face?" And the only animal I could think of is the horned owl. While generally, the owls are more brownish or reddish, they can look gold if they're the right combination of red, light brown, and white. That said, the "light of the dawn" is actually a lot more reddish than gold. So it could either be an exaggeration of the description of his actual color, to make it sound more noble, or his colors themselves were exaggerated more towards gold. Either explanation works.

As for the wings' shape, while you might say they look more bat-like than bird-like, I want to point out something interesting. They could have gone all-in with a bat look by altering them just slightly so that they appear more like bones with flesh stretched between. But instead, they left it more ambiguous. They specifically went without the bone-like extensions his old wings had. So it could be that without the ability to create feathers his magic attempted to just recreate the span of his wings with flesh. Gross, but an acceptable explanation. (Edit: the point about the wings also applies to the dragon theory, since dragon wings are essentially the same as bat wings)

The most striking aspect that made me believe he was specifically a horned owl, and not an eagle or similar bird of prey, is his face. The way the headpiece comes to a point at his nose looks very similar to the pattern found on most owls. And of course, the horns are similar to the "horns" (the feathered ear-tufts) such owls have. Albeit, aside from the fact he has horns, the shape is entirely different. But that can be explained by the imperfect nature of his form-- more importantly, there's isn't a different winged, "horned" animal with horns or anything that look anything like the angled and exaggerated shape of Aatrox's. And a bat doesn't fit, because the downward-pointed shape of the design is a direct contrast to the upward-pointed shape it would take if it were attempting to mimic a bat's snout.

For the record the fact that one of the Ascended, Cebotaru, had the appearance of a wolf means that the animal forms of the Ascended are not strictly tied to Egyption mythology. So any animal is fair game.

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As for the other two Darkin, I do like the idea that Rhaast was a shark. Though the shape of his horns isn't really reminiscent of a hammerhead's. While the fin-like parts on his forearms are evidence towards this idea, the fact his horns are in fact two and not one that branches in a T shape hurts that idea a lot. I think it's more likely he was a bull or a moose. Varus, I won't even get into. Because he didn't receive the VU he should have to coincide with the change in his lore. He completely lacks anything to make him appear like a Darkin, and that's solely due to their decision not to change his appearance to match the story. So any speculation about him would have no basis at all.

I for one would really love if they made skins for each of them where they've reclaimed their old forms. But perhaps that actually needs to wait until the Void fully breeches the rift between dimensions. My hope is that they'll do what they did with Gangplank, and during a big event make their Darkin forms skins (and give them for free to anyone who owns the champ) while swapping the base skin for the Ascended form. With Azir's return, it's possible that he could find a way to return them to their previous forms. Then I'd finally have an answer to this burning question.