I don't think some horses are genetically crazy, but I do know for sure that many horses have genetic diseases that can appear like the horse is "genetically crazy."
For instance, my personal gelding is wonderful when he's wonderful. He's relaxed, responsive, and very willing!
But he has a muscle disorder than causes his muscles to spasm and causes him a lot of pain on a pretty regular basis. When his muscles hurt, he's jumpy, spooky, and there's literally no way to "get through to him." On those days, he'll run you over without a second thought - something he would never, ever, think of when he's "in his right mind." The only thing that works is to read the signs and put him away because the day is not going to get better.
He's clearly confused in the first video because he had never been ridden without a bridle before, but he was trying his heart out for me. He didn't throw any kind of crazy tricks, nothing to make you think that he was anything other than 100% reliable.
And then the second video, he's just different. Who knows what spooked him [nothing??] but he got a littttle nuts. Looking back, it was probably some kind of muscle spasm or some other kind of "oh my gosh, things suddenly hurt so bad!"...but who knows.
I don't even ride him anymore because those moves were becoming so common-place. I never fell off, he always had to go right back to work [he never ever "got away with it"], but he kept telling me that something wasn't right.
Nobody quite knows what he has yet [he definitely has "something" but it hasn't been named yet], but we're lucky enough to be working with a research group that is dedicated to figuring out why and how these myopathies come about.
My gelding's issue isn't the only one out there that can present like this. There's PSSM1 that is typically early onset, but there's also P2, P3, P4, P5, and Px which are all later onset [ages 8-15 is typical to show the first signs] and display like a horse "suddenly going nuts" at a certain age. P2/P3/P4/P5/Px horses can all be wonderfully normal, delightful, mounts before onset and then something happens and it's like a switch gets flipped - they suddenly start getting jumpy, start bucking, bolting, etc. They can be back to their perfect selves the next day, and got nuts again the next week.
They aren't "genetically crazy" persay, but they do have a genetic disease that they can and do pass on.
P2/P3/P4/P5/Px are all diseases dealing with protein synthesis. Many affected horses can return to some semblance of their normal, perfect, selves if they are given an appropriate "complete" protein at adequate levels.
The freak out are literally caused because their bodies have an increased need for protein and the protein in their bodies is not adequate for the level of work they are being asked to do - if they are asked to work beyond the amount of protein "fuel" they have available, their bodies will literally start breaking the horse's own muscle down as a source of protein. Talk about a MAJOR charlie-horse!
That muscle break-down causes an incredible amount of pain [obviously] that only lessens with the cessation of work, unless they've been worked to the point of tying-up.
If P2/P3/P4/P5/Px horses have adequate levels of protein "fuel" in their bodies, they are able to work without pain.
Some horses have multiple myopathies together which increases their need for protein even more, and so forth.
Part of the muscle myopathy research is researching where these other [P2/P3/P4/P5/Px] genes came from. Most of the current research is focused on stock breeds.
And you know what? Many of those stock horse lines that have always been thought of as "difficult" or "cold-backed" or whatever have also come up with a significantly higher incidence of P2 and P3. So far, it's been similar with Thoroughbreds and Px - difficult TB lines = higher incidence of Px. The crazy Arabian stereotype? Turns out that a lot of "crazy Arabians" also have P3 and/or Px!
There are new discoveries everyday, but so far the evidence is mounting.
Anywayyyyy, I really don't think it's as cut and dried as "genetically crazy." I think there might be "genetically in pain" [and I agree that those horses should never be allowed to reproduce], but genetically crazy - no.