If you're mounted and forced to make a melee attack, do you attack your mount?

Question

I'm a new DM running a 5e combat encounter in which the NPC attacker is mounted on a Riding Horse.

One of the players' characters has just cast Confusion, which indicates that

Each creature in a 10-foot-radius sphere centered on a point you choose within range must succeed on a Wisdom saving throw when you cast this spell or be affected by it.

Both the rider and the mount are creatures, so they independently roll; both failed. Now, one of the potential effects on a d10 roll of 7 or 8 on upcoming turns is:

The creature uses its action to make a melee attack against a randomly determined creature within its reach. If there is no creature within its reach, the creature does nothing this turn.

It's not clear to me whether the rider would be confused enough to make a melee attack against its own mount.

Also, given the mount is not independent, it is not permitted to make an attack action, so I would assume it does nothing in my particular case, but I am curious about whether, in the case the mount were independent, it might attack its own rider.

Does a mount count as a "creature within its reach" of its rider? Is the rider a "creature within reach of a mount" if it is acting independently?

Answer 1

The rider can attack the mount

The mount is a “creature within reach” so is a valid target. Note that the actual creature attacked is determined randomly.

The mount can attack the rider

While a controlled mount’s action options are limited, this does not stop the mount from attacking if this result comes up. There are 2 reasons for this:

1. Specific beats general: the Confusion spell is a more specific effect than the mounted combat effects.
2. The spell causes the mount to “[use] its action to make a melee attack” - this is not one of the ‘normal’ actions a creature can choose from the combat rules nor is it an action from the stat block of the creature. It is an action type that the Confusion spell creates¹.

Bonus: if the rider is forced to move by the spell, they dismount first

That’s because the spell causes the creature to use “all its movement” - not its mount’s movement (and see this answer where the rider is forced to move by dissonant whispers - similar to confusion in that it is a mental trigger).

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¹_{While there is a standard list of Actions in Combat, note that "When you take your action on your turn, you can take one of the actions presented here, an action you gained from your class or a special feature, or an action that you improvise." Several spells, as class features, allow you to take 'special actions', defined within the context of the spell. Here, the 'confusion action' gives the controlled mount the ability to attack. As another example, the Call Lightning spell grants you the ability to call down lightning as a special action.}

Answer 2

Yes, the rider can end up attacking his mount.

The mount is a creature within reach of the rider, basically by definition as they share a space and the mount is a creature. Thus, it must include the mount as an option for the randomly-picked target of a confusion effect. Nothing in either the Mounted Combat section or in the spell say otherwise, so that's the only clean reading, at least by default.

The mount cannot make attacks while being controlled, so it does nothing in that case. Although there is a colorable reading that the spell is more specific than the Mounted Combat rules, and so the mount attempts to kick or bite (depending on what it has available) someone within reach, which might be the rider.

Note that this is actually fairly well aligned with reality, where crazed/badly-trained horses attack their riders with some frequency, and where a misaimed strike from a melee weapon can absolutely hit the horse (especially with a sword or axe).