Bats like warm places to roost - and sometimes they'll pick the rafters of a barn. Fortunately, bats are actually good to have around horses. (They can carry rabies, but it's fairly rare).
One exception is that in Australia, flying foxes can carry Hendra, which can infect horses - you don't want them in your barn or apasture.
Normal insectivorous bats, though? Good guys. They're handy for insect control and because they flap around at night, they aren't likely to spook your horses (or you).
Ironically, horses are more genetically related to bats than they are to cows!