Upon dismounting a horse (and boats, to a lesser degree) after riding for a significant distance or period of time will cause the horse to disappear. The horse will be completely invisible and cannot be targeted for interaction (mount, hit, feed). The horse will still make noises and exiting the world and re-entering will resolve the issue. The horse will re-appear around where it was dismounted, but is still susceptible to the issue.

How to recreate:

Ride a horse for around 1 kilometer or 4-5 minutes

Dismount horse.

If it does not disappear, ride for longer intervals until it does.