What do you mean, let your horse go?

In ancient times, generals rode horses to fight. When confronted, they all controlled their horses. Once a duel was about to happen, they urged them to fight each other immediately. So I used "bring it" as an agent, accepted the challenge of the other party and told the other party that I was ready and you could die.

Letting go of your horse means getting ready for battle.

It also refers to actually releasing and herding horses.