Here’s the thing- if you are looking for a Short Stirrup pony, I do not think this is an unreasonable requirement. Children need to learn to keep one leg on either side of the animal while it is behaving itself before they graduate to animals that are more likely to misbehave.
If you are looking for a trail horse for Grandma, unless Grandma is Betty Oare, I do not think this is an unreasonable requirement.
I think the average trainer understands that such animals come with a price tag, or with major trade-offs (like: one of the best “unicorn” Short Stirrup ponies I know has advanced Cushings and only has one eyeball, so she has some handling requirements to make her environment safe for her to navigate.) The average parent googling for Little Sally’s pony does not understand that ponies are hellions, does not understand what she is asking for, and does not understand why her requirements require an expensive history of training and preparation. It’s not a seller’s job to educate the parent- hopefully the parent is looking with the help of a professional or an experienced friend- but sometimes you watch that education happening in real time. So: how much of what this thread is complaining about is coming from professionals (or educated amateurs or juniors) versus Little Sally’s Mom looking, or Teenage Sally looking?
I have seen quite a few “ISO unicorn” posts from trainers- but every single trainer is defining what
unicorn means in context and talks about what they’re willing to look at in exchange for the must have unicorn traits. This includes, yes, it must be priced like it’s 2015 and can’t spook, buck, or rear. But it can be an off-size, it can rack instead of trot, it can require difficult maintenance and management requirements, it can be 23 years old and the prospective buyer will provide vet references and describe its retirement plan in detail upon demand, and it will have regular rides with a pro or a good junior to ensure its horn stays on. I believe they are successfully finding their unicorns.
By the way, I own a “unicorn” and I was Teenage Sally looking on DreamHorse, so I think I did a pretty good job there. He was a needle in a haystack 15 years ago when we bought him and his combination of common sense, sense of humor, training, and athleticism would make him a unicorn in this market, too. I got him by knowing the holes I could live with (his were his height, step, way of going, experience in my discipline) in order to get my non-negotiables (good brain, competition experience in an adjacent discipline, could teach me to his level.) And you can’t have him.
On the way to get him, I tried one who was simply the ugliest horse I’ve ever seen in my life but also malicious on the ground and failed the vet in ways my vet didn’t think that we could maintain effectively (but he was beautifully schooled, because you have to be when you have a head like that;) one that reared with intent; and one with a nasty bolting problem of the type where his brain shut off and he had no self-preservation. The second two were being advertised as children’s horses. So I also think @AltersAreUs is making a fair point about why “horse dealing” has always had connotations.