A single rear leg kick from a horse can easily break your skull apart, snap your neck or spine, cave in your entire chest, snap your femur, shatter your hip.

One good connecting hit and basically 99% of people are dead, paralyzed for life, or at best, extremely seriously injured and needing immediate hospitalization.

An unarmed human is not going to win an aggro'd fight with a horse in some kind of cage match scenario, unless its very young, very old, or quite sick or already injured.

Horses can be very dumb though: In a more real world scenario, you may be able to spook them into breaking their own legs on uneven ground, or even just running off of a goddamned cliff or into a tree or some other obstacle.

Alternatively: Spend the hour of prep time getting horse feed and poison.

Don't fight it, befriend it, and then betray that trust.