Can mounted creatures be attacked instead of their mount, and which character(s) in mounted combat receive attacks of opportunity?

Question

This is a question I brought up with my group and although I have made a ruling, I'm hoping to get a more authoritative answer (Sage Advice perhaps?).

The rules for mounted combat don't seem to specify how targetting works with the Attack Action. I've summarized the mechanics from PHB 198 below, with the final rule italicized because it is the subject of this question:

A mount is:

• A willing creature
• At least one size larger
• has appropriate anatomy to ride

Mounted and Dismounted:

• If within 5 feet of a mount, mounting or dismounting takes half your movement speed
• If mount is moved against will or you are made prone, DC 10 Dexterity or become dismounted and prone within 5 feet of mount.
• If the mount becomes prone you become dismounted within 5 feet of the mount. You become prone as well unless you spend your reaction.

Actions and Movement:

• A controlled mount must be a trained or domesticated creature, and they match your initiative while mounted. You may move the mount using its speed, and may only have it Dash, Disengage, or Dodge when it acts. Controlled mounts may move and act on the turn they become mounted.
• Independent mounts must be intelligent creatures and act independently, retaining their initiative order, movement, and full ability to act (presumably actions, bonus actions, reactions, etc.)
• If a mount provokes an opportunity attack while mounted, the attacker can target the mount or rider.

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The targetting rules of an Attack of Opportunity is specified, but can an Attack action also target the rider when only the mount is in range?

The Mounted Combatant Feat suggests that by default attacks may optionally target the mount, but leaves it unclear if the rider may be targetted whenever the mount may be targetted:

• "You can force an attack targetted at your mount to target you instead"

Answer 1

In general, riders and mounts can be targeted like any other creature

The rules for Mounted Combat don't specify how targeting works for the Attack action (or any other action) because targeting riders and mounts is no different that targeting any other creature, with the exception of opportunity attacks.

Whether you can target either, neither, just the rider, or just the mount has nothing to do with their status as rider and mount, rather it depends on many other circumstantial factors, the most common of which are by far range and total cover.

From Making an Attack:

Choose a target. Pick a target within your attack's range: a creature, an object, or a location.

From Total Cover:

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

For example:

• If your attack has a 5' range and you are a medium creature standing at the feet of a T-Rex, then you can target the T-Rex, but you probably¹ can't target its rider.
• If you can see a rider peeking over a wall, but its mount has total cover, then you can target the rider with an attack, but not the mount.

Opportunity attacks

The rules for Opportunity Attacks state that:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.

And that:

You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.

The rules for Controlling a Mount also state that:

if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount.

In short, a rider never provokes your opportunity attack when it moves using its mount's movement. But if a mount provokes your opportunity attack, then you can choose to target the rider instead of the mount, despite the fact that the rider did not provoke said opportunity attack.

However, this does not mean you can target a rider regardless of all other factors such as range or total cover. For example, opportunity attacks occur retroactively just before the target leaves your reach, but if the rider was never within your reach to begin with, then you can't possibly attack the rider.

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1. The height of creatures and the location of a rider atop a mount are often not well defined so there is room for DM adjudication.

Answer 2

An Additional Note on Opportunity Attacks, Highlighting a Major Advantage of Mounted Combat

The other answers seem to have the essence of your question covered: a creature taking the attack action can directly target rider or mount. This might seem like a simple answer, but your confusion with the rules is entirely understandable: why are the rules for opportunity attacks against mounted combatants written that way? In this answer, I explain why. It turns out that the fact opportunity attacks against a rider must be made vicariously through the mount creates one of the most advantageous benefits of mounted combat in 5e.

First, it is important to note that a creature on a mount is not moving by virtue of their own movement. From the PHB, 195:

You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.

Given the above, it would initially seem to be the case that a creature that is using a mount to get around cannot be targeted by an opportunity attack. However, as you have noted, the rules for a mount indicate that opportunity attacks against the mount can be targeted against either the rider or the mount. So the rider can be hit with an opportunity attack, but only vicariously through the mount:

In either case, if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount.

This might seem like a needless circle of rules, but it actually reveals what is perhaps the biggest advantage of mounted combat: the ability to ride in, make an action, disengage, and then ride away. Note that the mount "has only three action options: Dash, Disengage, and Dodge." The disengage action in this case is really special. Why? Because as we've established above, opportunity attacks cannot be made directly against you; instead, they are made vicariously through the mount, so if your mount disengages, opportunity attacks cannot be made against either you or the mount. In short, a mount allows you to disengage for free.

Answer 3

So long as a the rider and mount are both in range you may attack either one.

"The targeting rules of an Attack of Opportunity is specified, but can an Attack action also target the mount or rider when the mount is targetted?"

I don't think you meant to add "also" here. If you have already chosen to target the mount you cannot later choose to target the rider. I'm not sure if that's what you meant to ask.

The reason why the Attack of Opportunity rules are specified is because the rider is being carried and not moving of his own accord and there would be numerous arguments about "who was moving" and "who presented the opportunity". This is simply a clarification to remove arguments.

For your second (or maybe 3rd?) question: Mounts do not get bonus actions as stated on page 198 of the PHB: "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." No bonus actions are listed here and the text is very clear these are the only options.