I would want to be the middle person in a 3some but make that into a sex machine.
A horse dildo that would be able to be heated, flared, and have the ability to shoot cum. This would be programmable for all settings, including the thrusting speed.
A mountable mini mare that is heated. Would also be nice if it could contract or apply suction to pull you in. Auto lubricating would be perfect because I wouldn't want to break my immersion to put on more lube.
A full vr headset which would be able to let me see a scene of whats going on. Would obviously include the mare than I am fucking and the stallion I am getting fucked by. I would also have custom sceneries ranging from barnyard to central park. There would be simulated horse noises being played on headphones that have surround sound so I could tell the difference between the horses.