Hi, Linda --
I am not for sure I'd even bother insuring a horse for only mortality.
People who do so need to know that if the insured horse is injured or sick and regardless of how expensive the recommended treatment may be, if it's not done, that policy will not pay off if the horse passes away.
Right now, and since 2005, I have insured Destiny and DunIT through GenCorp for both major medical (each $7,500) and mortality ($17,500 combined). The annual premium is just over $1,000 but is worth the peace of mind it gives me. I think it's around 3% for the mortality and $250 each for the major medical rider.
Good luck,
Jill