At first I was, because running around was pretty boring, but honestly horses would kill a lot of the threat of the world, and long-distance travel can be handled by ships and portals anyways. Assuming that horses couldn't go through portals, I wouldn't have a use for one even if I had one. Maybe at most it would replace my cart for moving resources from my dock to my crafting areas.

That said, if we get more interesting maps that have more landmass, I could see a horse being the replacement for a ship in a mostly-land-based map.