Word To Use Today: hipparch.

 A hipparch is, sadly, neither an arch designed by an achingly trendy architect, nor a system of government by horses.

As anyone who's read Gulliver's Travels will know (the whole book, I mean, not just the bit featuring the giants and the little people (I think we're probably allowed to say little people in this context)) rule by horses might be a rather benign and marvellous thing:

Gulliver Taking His Final Leave of the Land of the Houyhnhnms by Sawrey Gilpin

but, as I said, the horses weren't hipparchs, because a hipparch was a commander of cavalry in Ancient Greece:

There were never a lot of hipparchs - cavalry numbers were small because keeping horses was expensive, and also because without much in the way of a saddle and nothing in the way of stirrups, they couldn't actually do much fighting without falling off their horses.

Still, fighting horsemen could throw spears and draw bows, and get places quickly, all following their hipparch.

Word To Use Today (though I can't imagine why you should need to) hipparch. Hippos is the Greek for horse. -arch comes from the Greek arkhein, to rule.