If you go with your horse far away from other players and then come back, those players won't see you until you dismount the horse.
Related issues
Comments
migrated
migrated
This problem is still in Minecraft 1.9.2 and also in Minecraft Realms
migrated
This ticket is resolved as duplicate, no need to comment here anymore.
The player is still visible standing where he last mounted the horse. He won't ever traverse, but will still look around and gesture (placing blocks, attacking.) If the player dismounts the horse, they appear to zip to where he dismounted.