Well, I've pondered long enough and I finally decided to try at the skeleton of a Pegasus. Reference pictures were chosen randomly from the web, so I don't own them. This ideal is purely my own, I do not claim that it is THE skeletal and muscular physiology of a Pegasus. This is just explaining how I see fit to the Pegasus' state. Please Do NOT Steal! You may use it for your own reference, but do not claim it as your own....

Part One of Pegasus Physiology (Skeleton)
I. Skeletal structure & Muscle Setup
II. Flight
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I. A pegasus would of course have the same skeleton as a horse, but with a few differences.
Noting the Scapular. Instead of the wing connecting right off the Scapular, I've extended it so the wing
joint would be more near the center/point of gravity (green dots) on the horse. This would give it a better
weight distribution.
For a pegasus to actually fly, considering it's weight, it might have hollow bones like a bird would, but with
strong wing muscles around the wing humerus, radius, and ulna.

II. Like the horse, the pegasus would use its legs to manuever as the Center of gravity would help its weight
distribution. However, instead of running on the ground, it could use its front legs to steer. Say, it was
standing on the ground and rearing, the Scapular would tilt back towards the ribs.
If it was running, the movement of the front legs would power the wings as they flap, like a duck taking off
from water, I guess), they need that head start.
It's not like the pegasus can just jump straight up and start flying, I can't see that happening.
Possibly, a few steps into a gallop it could take off. By using its legs to steer, it could gallop through the air,
that way the rhythm of its gallo would keep the wings at a steady pace as they flap.
To glide with the air flow, the legs can beheld a certain way so the weight of front and rear are equal.

As I said before, this is a completely original idea. Many of the reference pics I used were ramdomly chosen from the net, I do not claim any of them.
I might get more sketches of the pegasus physiology ideas later, but for now I need to think more on it~