While probably entirely inaccurate, here's what the Somalis themselves think their greatest leader looked like:

No, not the chap in "Black Hawk Down":
Sayyid Mohammed Abdille Hassan, the so-called "Mad Mullah."
In the absence of any actual proper info (I have a few books on Somaliland, but none show a photo or illustration of the Mullah, or any other leader for that matter), when my Somali project comes off the back burner I'm planning to go down this fanciful equestrian statue route if I need a mounted leader (And why not? Ancients wargamers get their uniform details from dubious statues, columns and wall-carvings, so why not Colonial gamers too?).
My thoughts have been heading towards Perry town. I was thinking maybe one of these fellas with their coloured patches filed off:

Or maybe old Osman himself. I mean, a Beja 'fro ain't all that different to a Somali one, if you squint:

So nothing concrete, but at least something you can 'get on the table.' After all, the Brits called the Mullah's boys "Dervishes" too. And a Dervish is a Dervish, right?
For a leader on foot, Allen has it. Allen's advice is always worth following. Well, in the area of history, gaming and miniatures, anyway. Don't blame me if you get burned on the stock market after listening to him, I can't vouch for that.