In addition to designing new creatures for the Aerie, I'm also planning on revamping old ones. Dragons, gryphons, winged horses, etc. And anything with six limbs and this kind of body plan is going to be related. Dragons will be the oldest of the hexapods, with creatures like wyverns, wyrms, winged horses, and gryphons stemming off of them, losing limbs as necessary.
Wyrms are very old, specifically lindworms (serpentine dragons that possess only arms). For these varied creatures I'll need to create multiple iterations. Some will be more serpentine, while others will resemble modern "drakes". First up is an amphibious lindworm, this guy heralds back to medieval stories of wyrms living in wells and rivers. They spend about as much time on land as they do in the water. For these and all wyrms, I want them to look almost prehistoric, like the ancestral mammal-like reptiles, (I'm thinking of creatures like Lycaenops). They're likely to be venomous and may kill their prey via constriction.
The winged horses are not horses at all. They're based on the idea that early civilization easily mistook strange animals for something entirely different. For example, a manatee becoming a mermaid in the eyes of a sailor. These creatures will be rare herbivorous hexapods, having branched off from dragons very long ago. Their large guts (needed for digesting plant matter), make them too heavy to fly, but they retain their wings for gliding purposes. As the winged horses live high atop cliffs and hillsides (much like goats), their wings come in handy for quick getaways. If they are cornered or a predator ventures too near, they simply leap off the cliff and sail away to safety.
Though the majority of the animal will be covered in feathers, the wings will be leathery like those of their ancestors. As there is no reason for a dragon to lose all their wing membrane and then regrow the wing composed of a new material. The wings however look very much like that of a bird, specifically an albatross in profile because of the feathery vanes the webbing forms.
Right now I'm leaning toward the sketch in the bottom left corner. I'll make more sketches based off that one and continue to refine the creature design. The goal is that this animal must look powerful like a horse while still being lightweight and distinctive. It evolved from a carnivore after all, and while it should still be reminiscent of its namesake, it must also display hints as to its original nature.