Lately I've been trying to flesh out more aspects of my headworld, specifically... anatomy. I've always been a biology nerd and studied anatomy to draw animals, but the concept of making up muscles and bones absolutely terrified me. Still, I really wanted to make my critters more plausible, so I decided to give it a shot. Now, after spending over a week straight with my nose buried in anatomy books and having overcomplicated muscle names burned into my retinas, I think I came up with something decent.
My headworld is a sort of alternate earth. Its evolutionary line followed very similar branches(reptiles, mammals, birds, etc), but with twists here and there. A major one being that all vertebrates after fish came from a common hexapodal ancestor.
Although most lost the third limb pair in favor of easier locomotion, some lines still retained them and adapted those limbs to specific functions. In this example, a clade of omnivorous felinid animals adapted for climbing and foraging, using its highly specialized third limb pair to climb as they pick food such as fruits and insects.
Over time their climbing limbs would also be adapted for gliding, which allows the animal to chase after small prey and also make a quick escape from predators. The skin membranes are supported by cartilaginous structures they can fold, similar to a pterosaur's wings.
That's the basic idea of how dragons would come to evolve in my headworld. These animals would gradually adapt their arms further into bat wings and develop powered flight. And for extra context, in this world true dragons are mammalian, while winged reptiles are considered false dragons.
Hope you enjoy my trip into insanity. Now back to my musty cave.