A five-gaited horse "racks"....Now, I always heard it was a man-made gait... A rack in a 5-gaited horse (aka Americal Saddlebred) IS man-made. It has to be taught. It is in-born in the breed "Racking Horse", and in Tennessee Walkers, Rockey Mountain Horses, Kentuckey Mountain Horses and others I'm sure.
Can you try to send a digital film of one of your horses racking? I'm not sure I'd be savvy enough to recognize it, not sure I've ever really seen it!