You fuck her in the ass for a few minutes. Her ass is nice and warm, and very tight, just how you like it. But eventually the position you are in become uncomfortable for your cock, and you withdraw, with a nice, wet, sloopy squelch.
That's when her vaginal muscles began to convulse inwards. 'Finally,' you think to yourself.
She ends her motion, and allows her strong muscles to hold you in place. She steps away from the tree, bringing you with her. 'Wow,' you think, 'This girl's got some muscle.' Your legs were still deep inside her, and you seemed to defy gravity as you stayed put above her rump. Nothing but her vaginal muscles were keeping you from falling out of her pussy and hitting you head on the ground.
Suddenly, she slowly bends down, as if she was going to sit down. Then you realize that she is sitting down...and that she was going to sit on you! You struggle to get out in time, but her pussy muscles held you firmly in place. And before you knew it, your back was on the ground. You let your arms drop down, spread out like an eagle. The mare sees this, and takes her chance. She places a back hoof on one of you hands, and before you had the chance to try and get it out from under her hoof, her other back hoof was already on your other hand. Strangely, for a horse of her gigantic size and weight, she seemed light as a feather. There was no pain as her full weight was on your hands. And all you could see now was her big rump, and the big, round tight pucker that was her anus. But she wasn't done yet. She resumed her masturbation session, using you ass and legs as her dildo. She moved up and down, her lips engulfing your entire member as she came down before she went back up. She started out slowly first, letting you savor the feeling of her gorgeous lips sliding over your whole member before withdrawing it briefly. Then she increased her speed. She began to move up and down faster and faster. Your member totally inside her for a split second, and withdrawn for a split second, before going back in. The sensation of a horse using your body as a dildo to masturbate, coupled with the feeling of your member being repeatedly engulfed by her lips finally made you cum. It was the biggest orgasm of your life, but your seed quickly mixed with her sex juices, which were beginning to flow out of her cunt steadily as she sped up her rate of masturbation. Her juices coated your entire body, making you slick, which in turn made her masturbation of your body more easy, and faster. All the while she is doing this, her head is held high, and her tongue hund out of her mouth. Then, the floodgates opened, and you could feel the shear power of her orgasm as her hot and sticky juices washed over you. It seemed to never stop flowing, and you only had brief moments where you could gasp for air. Then, it stopped. You could feel the pouding of her heart around your legs as she panted heavily from the pure pleasure she experienced. And then, she resumed to sit down on you
She continued to bend down backwards, and her big rump kept coming closer and closer to your face as she sat down on you. She removed her hoofs off of your hands as she bent over.
Your butt slid into her depths, and she nickered in pleasure as it streched her even farther for just a few seconds. Then your hips disappeared into her, and when she was fully sitting down on the ground with her anus directly in your face. And then...