I presume you're looking for mortality/major medical/loss of use type insurance?
I currently have mortality insurance with a colic surgery rider on it through Wilkins Insurance (equine insurers). I don't insure for the full value, or I guess you could say I self insure the rest of the value. They have a $6,000 minimum insured valued per animal, and you pay 3.2% of whatever the insured value is for the mortality. I've carried major medical in the past with another provider, and did use it, but decided it wasn't worth the additional premium.
Have known several people with Wilkins that have filed mortality claims with Wilkins and been very happy. One friend is awaiting payment. Horse went in for colic surgery, subsequently coliced a second time post surgery while still at the surgical facility and was euthanized. Not sure how much out of pocket she will be, since the colic surgery alone (with recovery) was around $13,000, and she too had the horse underinsured.
I'm not aware of any farm policies offering mortality policies on horses, but learn new things everyday. I've been able to add riders to farm policies in the past that would cover mortality on horses in the event of a lightening strike very inexpensively, but that's all the insurance I was able to scare up on a farm policy.
Good luck.