I can't imagine coyotes would take on a full-grown mare, especially with another horse around. I live in an area with lots of coyotes, blackbears, and even a few wolves, and I've never heard of any of these wildlife taking out a horse. In fact, I'm sure our horses chase any coyotes that come through their pastures - they certainly like chasing dogs that are around the same size.
I also know people with minis in their herds, that are left out overnight, with no issues.
I wouldn't worry about your mare, even with the bad eye.