A horse has to be far more sexually stimulating than that.
I don't know where to begin with everything else in your post. I've not had many behavior or psychology classes. So maybe you're right, even, but I doubt it (protip: if you reference Freud, you're not getting any points; I can PM you why sometime tomorrow, if it's actually relevant to your arguments).
That said, the part I've retained in the quote is wrong, because horses simply don't vibrate at anything resembling that frequency. The closest thing you'd find is a heartbeat, and that's still only 180 bpm even at the highest rates during exercise - my research into typical vibrator frequencies has, thus far, been unproductive however. Maybe I overestimate them, but I would be surprised if the range overlaps. But that's assuming you could somehow sense it (you're almost certain not to unless you're bareback riding, naked, and have a very sensitive sense of pressure). Even if it were fast enough, and a rider could feel it, I'm still doubtful that the force would be applied in the right way; it'd be entirely perpendicular to the surface, which I am
pretty sure is not helpful for a vibrator (somebody who's actually used one can enlighten me).
Realistically, the only shocks the rider is going to feel are from the actual movement of the horse, which will not even approach the kind of stimulation you're talking about. It could more accurately be compared to sitting down repeatedly on an oddly-shaped chair, unless I grossly misunderstand the mechanics.
EDIT: Oh, and just to put the spoiler title in perspective, the first thread I ever started on these boards was entitled "Dwarven Sex: Or, the Making of Gimli", so you should know I do not think highly of this one.