I am honestly baffled that you've got yourself so convinced he deliberately jumped on the rider. Not when there's as much momentum and speed as there is going over a jump. You'd better have it on film.
All I'm saying is, that IF the action was deliberate, that's the horse's "first bite" (that's a term of art - in this case deliberately jumping on somebody) and another bite will be actionable.
We don't have any evidence here that would hold up in a court. IF the story shook out as I noted earlier, it would be a case. But we don't know. So I'm NOT "convinced" of anything, as you put it.
But it's a cautionary tale for all of us to be careful about horses with known problems, and always warning anybody who rides them. THAT's the validity to us here, not whether or not some hypothetical person in the future could sue.