We have a Gnome Fighter and a Dragonborn in our campaign. The dwarf sometimes gets stuck in a makeshift sling on the back of the Dragonborn to shoot arrows at enemies in combat. So far, I have considered the dragonborn as a stand-alone mount, when AoO appear, and possibly provided the gnome with a lid.
Recently, the dragon received a horse. Now the gnome is on the back of the Dragonborn and the Dragonborn is on the horse. If the horse is a controlled mount and provokes an AoO, the attacker can attack the horse or the dragonborn. On mounts of the PHB states (198):
… if the mount triggers an opportunity attack while you are on it, the attacker can attack you or the mount.
That seemed easy at first, but then we realized that dragon BornNot the horse, is the mountain of the dwarf. However, the Dragonborn will never trigger an opportunity attack, as the Opportunity Rule rule states (PHB 195):
Nor do they provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.
In this case, neither the Dragonborn nor the Gnome can trigger an Opportunity Attack because they are not moving with their movement, action, or reaction. Only the horse can trigger an opportunity attack.
If the attacker can not attack the gnome in this case, the gnome is completely safe from occasional attacks.
When the horse triggers an opportunity attack, the attacker can choose between the dragon-born, the horse, and the gnome?