I'm on the fence with this one. I got mortality + major medical for my horse when I bought him, but let it lapse this year and don't think I'll renew it. For one, he's proven not to be the accident prone type (the horse I leased before him managed to accrue a $500 vet bill playing too hard in turnout, so I was a little wary of this) and I'm in a better financial position now to handle an expensive vet bill than I was two years ago.
If your horse is very valuable, then mortality would be a good idea. If you don't have a good emergency fund set aside for the vet, then major medical could be a good idea as well. You'll have to get mortality to get major medical coverage, but since mortality is based on the horse's value (usually around 3% of their declared value) it costs less for less valuable horses. You can also insure your horse for less than its value if you so desire if the mortality part isn't that important to you.
Also make sure you have liability coverage. Many renters and homeowners policies will cover horses for liability, so if you already have one of these call them up and make sure that it applies to your situation. (Mine also covers my saddles and other equipment that are kept at the boarding barn, which is nice peace of mind!) If they don't, then consider getting a policy specifically for horse owners or an umbrella liability policy that will.
“The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will.” - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon