I'd go with high quality house paint. Why would you worry about it being safe for horses? Do yours chew on the barn? I'm pretty sure paint is paint when it comes to safety- you don't want anything eating it.
When we painted our small barn we were told of Kilz Casual Colors exterior paint. It absolutely coated in one coat and has held up fantastically for over two years now. It is sold only at WalMart but don't let that keep you from buying it. The paint is unbelievable. We used the barn red, that you can buy by the gallon or 5 gallon, but they can mix any color. If the name Kilz sounds familiar it is because at first they only sold a primer product.
The other female in my husband's life has four legs
We were warned off of the barn paint at our feed / farm supply / hardware store, poor coverage, does not weather well.
Check if any of your memberships carry a discount. My ApHC memberhip offers a discount for Sherwin Williams. This discount can be 20 - 30 % off a gallon.
We just painted a barn and fence and used MBA (division of SW) house paint. Told the clerk what we wanted to paint and ask if they had something economical. He had four pails of a line that was discontinued, sold it to us for $75.00 for five gallon pail. This was normally priced at $289.00 per pail.
We used a Wagner Paint Crew to do our painting. What a time saver!