My Riding Stables 2: A New Adventure (UP)

When your horse is in the stable, one of the options you'll have is to groom it with the brushes. Simply run the brush on the horse until the dirt meter goes down - generally speaking, there will be dirt on the neck, the side, both thighs and sometimes on the horse's back. The game will tell you when the horse is fully clean. Once you've done that, you'll need to press start and select "return to yard" to get out of the cleaning minigame.
 
You'll also have the opportunity to switch between a hard brush and a soft brush. The soft brush is light brown and gets the dirt off more quickly. The hard brush is dark brown and still gets dirt off but raises the trust level very quickly, even if the horse is clean. If you find yourself in a situation where the horse's trust has dipped below the red line (meaning you can't take it out to ride), choose to brush the horse, then brush its side until the trust meter is maxed back out.