The bow used I think is fine. It may be a bit longer than the recurved bone bows of the Steppe Horsemen but it is not a long bow either. It is recurved and therefore appropriate for something out of Greek myth. The use of the bow from a horse is another matter. The stance of 90 degrees to the tareget is fine for archery practice or using a long bow for distance shooting but using a recurve off a moving horse...even if in a way you are the horse!....takes another kind of consideration.. That is why the character seems stiff. Wrong stance for the situation. I still think the horse's rear legs need to be firmly planted on the ground to justify the rearing action. In addition it would be better for the dynamics of the body of the horse were more twisted toward the prey bringing into a better position the human body for the shot. The human body can also be slightly twisted at abdomen/chest and shoulders to add a continuum to the horses left body twist and perhaps a slight forward bend as well. I have seen renders of riders of the steppe shooting their bows from horseback and there was much bending and twisting of the body into less stiff and awkward seeming poses. Motion and position change of the prey demands quick response w/follow through of the body positioning on the fly...or in this case....on the hoof. I think that is the dynamic missing from this image. I don't think you need to research more archery and bow use poses as much as you may consider researching horseback battle stances.