Somalis didnt do mounted archery for the same reasons Ethiopians didn't. Somalis either didnt have stirupps (the part you put your feet that connects to the saddle) or used the Big Toe stirrups just like them. By that I mean the stirrup was a leather loop that could only accommodate 1 toe and not the whole foot. You cannot aim without good stirrups that can hold your weight. To be able to aim a bow effectively on horseback, you have to stand up a little bit on your stirrups and keep yourself balanced and level while your horse gallops using your thigh muscles to absorb the shock of the horse galloping.
Heres a description of the somali saddle and stirrups.
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Here is an Ethiopian horseman using the same sort of toe stirrups that we used. These small and weak stirrups would never allow for this sort of maneuver. They were just there to make riding more comfortable so aiming effectively like mongolians would never happen.
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Another reason is because somalis didnt use composite bows like mongolians. We used self-bows (one solid piece of wood) like western Europeans did. To have a bow as powerful as a mongolian composite bow, they need to be huge like these Somali bows, and thats just too awkward to deal with on a horse. Thats why we dont hear about English horse archers they moved from bows like us to guns without developing a horse archer culture
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Btw you may find images of somali horseriders with whole foot stirrups online, such as Risalder Major Musa, but these arent native. For him it was given to him by the British.
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