The description of enemies abound (Xanathar's Guide to Everything, p. 155) states:

You reach into the mind of one creature you can see and force it to make an Intelligence saving throw. A creature automatically succeeds if it is immune to being frightened. On a failed save, the target loses the ability to distinguish friend from foe, regarding all creatures it can see as enemies until the spell ends. Each time the target takes damage, it can repeat the saving throw, ending the effect on itself on a success.

Whenever the affected creature chooses another creature as a target, it must choose the target at random from among the creatures it can see within range of the attack, spell, or other ability it’s using. If an enemy provokes an opportunity attack from the affected creature, the creature must make that attack if it is able to.

If you (successfully) cast enemies abound on a mounted opponent, what happens?

Since the mount is a creature the affected target can see, they should treat the mount as an enemy as well and I can't see any reason they shouldn't but I just wanted to know if I was missing anything.

If they do see the mount as an enemy, how do the rules handle them attacking the mount while still on it or dismounting from it?

A mounted opponent who fails the save regards all creatures as enemies, including the mount they're riding

The enemies abound spell description states:

You reach into the mind of one creature you can see and force it to make an Intelligence saving throw. A creature automatically succeeds if it is immune to being frightened. On a failed save, the target loses the ability to distinguish friend from foe, regarding all creatures it can see as enemies until the spell ends.

The direct effect of the spell is clear: if the target fails its save, it regards all creatures it can see as enemies. Assuming the target is riding a creature (rather than a Broom of Flying or Carpet of Flying or something), and assuming they can see, they would indeed regard their mount as an enemy as well.

There are no special rules by default for attacking a hostile creature you're riding, or dismounting one

By default, the rules don't generally handle such a unique case; especially since the enemies abound spell was added in Xanathar's Guide to Everything rather than appearing in a core book, I'm guessing the designers didn't necessarily anticipate the spell's existence when writing the rules.

However, there is a potentially relevant optional rule on DMG p. 271, under the Action Options heading, titled "Climb onto a Bigger Creature":

As an alternative, a suitably large opponent can be treated as terrain for the purpose of jumping onto its back or clinging to a limb. After making any ability checks necessary to get into position and onto the larger creature, the smaller creature uses its action to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target’s Dexterity (Acrobatics) check. If it wins the contest, the smaller creature successfully moves into the target creature’s space and clings to its body. While in the target’s space, the smaller creature moves with the target and has advantage on attack rolls against it.

The smaller creature can move around within the larger creature’s space, treating the space as difficult terrain. The larger creature’s ability to attack the smaller creature depends on the smaller creature’s location, and is left to your discretion. The larger creature can dislodge the smaller creature as an action—knocking it off, scraping it against a wall, or grabbing and throwing it—by making a Strength (Athletics) check contested by the smaller creature’s Strength (Athletics) or Dexterity (Acrobatics) check. The smaller creature chooses which ability to use.

The DM may choose to apply similar mechanics in the case of a creature affected by enemies abound that is mounted on another creature. Most mounts would likely not be able/intelligent enough to attack their rider, and thus would likely simply attempt to use their action to dislodge the rider if the DM decides to use these rules (assuming the rider attacks the mount first).

The character sees the mount as an enemy

As you mentioned, the mount is a creature, the character can see it, so it's now seen as an enemy.

How does the character respond is up to the character

If that's a PC, it's up to the player to role-play it. If it's an NPC, it's up to the DM how he will handle the fact that he's now mounted in something he sees as an enemy. He could stay mounted and attack it, he could jump away, scared, or whatever that character would do.

The spell does not tell us how the character has to behave, other than

Whenever the affected creature chooses another creature as a target, it must choose the target at random from among the creatures it can see within range of the attack, spell, or other ability it’s using. If an enemy provokes an opportunity attack from the affected creature, the creature must make that attack if it is able to.

So it has no obligation of dismounting or even attacking.

It's interesting to note that the creature does not see the character as an enemy, so, until something happens, the creature itself is as willing to let him keep mounting as it was before.

  • (wrt the last paragraph) Which also means that if the rider chooses to attack the mount, a good DM call would be to treat it as surprise and apply advantage on the first attack. – cpcodes Aug 2 at 16:04

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