It's a two-seater horse because two-person mounts are the new thing and ZOS want to bring out several different variations to appeal to different people.

Of course they could have put the same pattern onto a one-person mount, but chose not to because right now they're focusing on making more two-person horses.

It's best not to try to read too much into the names and flavour text on crown store items. They always tie into the lore in some way but places mentioned are often arbitrary, especially for mounts and pets. The fact that this one happens to mention Skingrad doesn't mean anything more than the Riverwood White Hen or all the mount descriptions which say they have to be tamed in a specific place or are awarded to followers of a daedric prince or whatever.