When a character with the Sentinel feat hits with an opportunity attack against an opponent with Mounted Combatant, does the mounted opponent need to stop moving?
The Sentinel feat (PHB, page 169) gives the following benefit, which is usually clear in its applicability:
When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn.
It seems a bit ambiguous regarding controlled mounts, due to the way movement rules work, and the way Mounted Combatant redirects attacks.
The relevant benefit of the Mounted Combatant feat (PHB, page 168):
You can force an attack targeted at your mount to target you instead.
The rules on controlling a mount (PHB, page 198) state:
The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge.
Suppose the scenario is as follows. Person A has the Sentinel feat; Person B has the Mounted Combatant feat, and is on a mount. On B's initiative, the mount goes, and B gets within reach of A and gets hit with Sentinel's opportunity attack as they try to leave. However, B redirects the attack from their mount to themselves.
While the attack of the character with Sentinel was aimed at the horse, it ends up hitting the rider. However, the rider isn't actually using their own movement here; the horse is. Does the horse/rider pair stop regardless, as they are moving together? Is there some weird interaction where the rider is stopped, the horse is not, and they perhaps have to make some sort of save to stay in the saddle? Or does Mounted Combatant negate this portion of the Sentinel feat, rendering it ineffective?