On 15Dec2017 2221, shadow at shadowgard.com (via tml list) wrote:

Somebody questioned if canine spines are suitable. *Equine* spines
aren't that well suited to riding.

Thats why saddles sit just behind the horse's shoulders, so as much of the rider's weight as possible can be transferred directly to the forelegs, and not to the spine. It's also why if you sit too far back when riding bareback many horses will attempt to remove you - it hurts and they don't like it.