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?
Best Answer
A mounted opponent who fails the save regards all creatures as enemies, including the mount they're riding
The enemies abound spell description states:
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":
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).