[RPG] Can you use your own movement separately from your mount’s movement

Given that you appear to have all the rules already, it seems that the problem is in interpretation. Given that, my view is :

1. Should the mounted PC be treated as a normal mount for a PC, losing his turn and becoming subordinate to the rider?

   • Answer: No. The mount is the same level and equal in power

2. If 1 is "no", do they share initiative counts?

   • Answer: No. They each roll initiative, and act on their turns. If one wants to move and the other doesn't, then the rider either dismounts (if the rider is the source of the move) or falls prone (chosing not to move with the mount). As a generous DM I'd allow an acrobatics check to avoid falling prone.

3. If 1 is "no", can the rider use his move action to make the mounted PC move, or force the mount to act by spending his own actions in any other way described by the Mounted Combat rules?

   • As above; if one wants to move an the other doesn't, then it's either a dismount or a fall-prone for the rider, depending on who's turn it is.

   • Per the comments; the mount should only be able to be move normally once per round - whichever commands the mount uses their move action to move, the other still has a move action but can't use it to move (unless it's the rider dismounting). Clearly other sources of movement (e.g. warlords) would still have the expected result - and the mounted combat rules cover that sort of thing already.

4. Can the mounted PC use the rider's base skill check bonuses as described in the Mounted Combat feat (e.g. a human with a pixie on his shoulder uses the pixie's Stealth modifier)?

   • If the rider has mounted combat and the mount is willing, then I would go along with this.

5. How would feats like Holy Steed, which gives a rider's mount bonuses to defenses and damage, be applied?

   • I would go along with this. As a DM, I'd much rather my players bunch up than spread out. A small bonus is worth the penalty.

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.

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.