Power word: HORSE
This spell acts on all entities in range, including the caster.
If the target is sentient, this spell transforms the target into a horse, similar to Polymorph. Unlike Polymorph, since this is a powerful horse, the target retains all its stat scores, abilities, and equipment benefits.
If the target is non-sentient, not a horse, and at least size Small, this spell is similar to Charm Animal with two additional outcomes. On success, the target is willing to do all actions germane to both its normal form and to horses, without you needing an additional Charisma check to persuade it. Also, if its base Strength, Dexterity, or Constitution scores are below that of a riding horse (16; 10; 12), they rise to meet those levels.
If the target is a horse, nothing happens. What did you think would happen?