I've seen some other riders, but they're mostly in the southern residential region. But I was playing in the town area today, (I converted the "park" lot into a residential ranch so I could have a rancher with many horses for my Sims to visit and ride) and saw some riders passing by the edge of town but not in the town proper, which remains a ghost town, and not the fun kind. And a couple of riderless horses from other households appeared on the outskirts near the general store, but didn't do anything, just kind of hung out over there on the road away from town.
I've also seen a rider or two in the far upper region neighborhood, but mostly just people walking...and ghosts too. Many ghosts on foot. Too bad, it would've been awesome to see ghost riders.