Neither I nor anyone I know has insurance on their horses. I have a friend who used to have insurance on her horses and one colicked and had to have colic surgery. The insurance paid for most of it, but if I understand it correctly, the insurance company dropped his coverage after that. So my friend dropped all her horses off the insurance plan. They are wealthy enough not to need insurance on their horses anyway.
I personally don't because I don't have that much excess income and my horse's aren't that valuable.....money wise. Emotionally they mean everything to me though.
I have lost two horses to colic but they were in their mid to late 20's and I don't even know if insurance would still have covered colic surgery for horses in their mid 20's? But I wouldn't have put my older horses through surgery anyway. So I guess I am taking a risk. I will treat them medically as much as I can within reason. But something big like colic surgery will likely result in euthanasia for a horse of mine.
Pretty much everyone I know is in the same boat. Except for said friend who can afford to pay out of pocket if she needs to.
I think it depends a lot on what kind of horses we are talking about. I would bet most valuable show horses are insured. I am in a rural area and most of the horses are just kept for pleasure and trail.