Bugs on my horse??????????

I brushed all 4 of my horses today I didn't notice any bugs. My filly had a couple spots where hair was missing. I just figures it was due to a nip from another horse. After I finished brushing her I hand jogged her around the field. When I dropped the lead for her to stop I turned & dropped with her, my eyes with her shoulder. I noticed some hay dust on her & went to brush it away but then notice it was MOVING!!! She was warm & these tiny bugs were all over her shoulder. They quickly disappeared as I was trying to squish them. What could these be? I've checked pictures of mites, fleas & lice & none of them seem to be what I saw. Any ideas??