To make an obviously fictional creature appear realistic, the creator should take into consideration the modifications of almost all of the body's systems as well as its' general ecology. This is what I'm trying to do. This is the companion piece to "Equavus - Skeleton". This is the best that I could come up with for a winged horse that can efficiently fly, run and survive. The ability to fly is not to be taken lightly; If their is no need to fly, birds will quickly lose the ability in just a few generations. So I approached this issue with as many refrences as I can. This is what I have: For the most part, all the muscles on the head and neck are nearly identical to a horses' comparative muscles. In fact, the first major changes aren't seen until the shoulders As is obvious, they have three sets of limbs - two pectoral and one pelvic. The spinal column has a double notch in it to hold two sets of muscles, although only the superficial muscles can be seen here. The muscles acting on the forelegs are both thinner and smaller then the muscles acting on the wings, as this is primarily an aerial animal. The pectoralis major is large and broad with the muscle belly curving in front of the inferior shoulder to attach to the humerus of the wing on the superior shoulder. All of the dorsal muscles that have insertion points on the pectoral girdles, such as the latissimus dorsi and trapezious, are split. Note the latissimus ventri that is just visible on the animals’ ventral side. This muscles’ insertion point is on the humerus of the forelegs and is the hexapeds’ weaker version of the quadrupeds’ pectoralis major. Like with birds, nearly all the muscles on the pelvic gridle have conjoined into two main muscles, with a few smaller muscles acting on the fibula and tibia. Like with birds, the equavus reproductive system is modified for flight. Unlike with birds, only the right ovary is used, while the left ovary withers and becomes vestigel. Males, however, follow the avian example more closely. Their testis, while normally very tiny, swell to hundreds of times their size during the breeding season. Similar to birds, all of the equavus' reproductive organs (with the exception of the males' penis) is held in the pelvic girdle, making the castration of domestic males next to impossible for most societies. Unlike with birds, and, indeed, more like marsupials, the equavus possess both a cloaca for excreting waste in the condensed frorm of creatine as well as a separate orifice for reproduction. And that's all I have for now. I know I made a few mistakes on the wing muscles, but I assure you that it is large enough to support the organism in flight.