No. The horse is not too thin. The problem seems to be that his "chest" (rib cage) does not go back (toward his stomach) far enough:







The head looks a tad small and the eyes are certainly not correct for a "normal" horse. Also, the front legs on the model look too thick from the front view:



As powerful as horses' legs are they are thin ... only bone and muscle, which is conforms tightly to the bone.