2 (or more) players on Realms. Player 1 mounts horse. Other players do not see player 1 mounted on horse, only see horse without rider. Other players see player 1 standing still where horse was.
When player 1 dismounts horse, other riders see image of player 1 suddenly merging with dismounted player 1.
Generally: mounted riders rarely visible on Realms, usually appear to other players as empty horse/invisible rider. Mounted players sometimes visible to other nearby players.