Get the crate near you and start the fun.
From Create Your Own Story
You need to remove the pain of your boner now, and you sure aren't going to take your time right now. The owners could be back at any minute, so best to get cumming now or face the music. You put the crate next to the mare's pussy, you get on top, and you undo your pants; having it be right around your ankles. Slowly you begin to insert your member into the horse, making sure not to startle her and hurt you in some way. Luckily, as you enter, the horse accepts it, even backing up slightly to insert more of you into her.
You almost fall back, but you hold yourself steady. You lightly grab her hips and begin moving yours, feeling her big pussy all around your cock. You go faster as time is of the essence, and soon you're at a normal pace. The horse stays still and even neighs sometimes like she's enjoying it. After a few minutes of thrusting, your heart rate is getting faster and you're shaking somewhat, perhaps realizing you're having sex with a horse and liking it. However, before you could enjoy it anymore, you feel that same old release, and you pull out of the horse just in time to spray your lovely fluids all over her rump.