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.
Related issues
Comments

Go to the main ticket, MC-96954, which is not resolved. This ticket is marked as "resolved" because it is a duplicate.
Bug is still present in 1.9.4. In what way has this issue been resolved?