I’ve insured 2 horses through Great American/Marshall & Sterling. I’ve found the coverage to be comprehensive and the rates very reasonable. I insured my then 15yo with them in 2015. His insurance value was less than $5k but we were able to get up to $7500 in coverage.
Current horse has major medical and mortality for insurance value >$10k. Premium works out to just over $100 a month and the policy has paid for itself nearly 10 times over this year alone. I’m still waiting for my new binder for our 2022 policy year, but there didn’t appear to be any exclusions or increase in premium on the renewal paperwork despite a hefty, complex claim this spring.
I know other folks in my area have had great luck with Markel, too.