I don't think Fescue is a good grass species for horses.  

If you ask a horse person, they will say they want timothy.  However, I don't consider timothy to be a good pasture grass.  Get past spring, and I don't believe timothy is very productive. 

IF you look at a horse pasture, it is most likely going to be mostly Kentucky bluegrass.  The way a horse eats, Kentucky bluegrass is the one grass that can withstand the way a horse eats. You might as well have at least some in the mixture you plant.