I was thinking the horned horse might be called a bicorn, but
the historical mythological creature already using that name is somewhat different that what I intend. It is also a perverse take on a unicorn, in that it subsists on a diet of devoted (or henpecked) husbands.
One other way my horned horse diverges from the unicorn model is that it's ram-like horns aren't capable of a lance charge like the TFT unicorn. But I would say they add blunt damage or impact to a charge attack, perhaps sending smaller foes flying through the air!