I was riding one of the lesson Horses at my barn and we were trotting pretty well and he started attack the indoor arena mirror. My instructor said "Iceman is just weird and he likes attacking himself". So I have to make sure to pull him off the rail a little bit when I approach a mirror. Lol
The farm I trained at in Colorado had mirrors all over the indoors (there were two at the farm). Each one had curtains. We stood a number of stallions there and would occasionally turn them out for a gallop in the bigger indoor. The stallions, if the mirrors were not curtained, would attack them, too. It is a territorial "space" thing.