Horses are animals with a very complicated response to anything , I have 3 max my young horse will pass JCB , Bus etc with out a blink of an eye , but a grid or bin he will make it known its something really scary,
then the next minute he will have his head in said bin, so no real logic, he prefers it when cyclists say something to let us know they are passing etc, he weighs 650Kg so can accidently crush most things without meaning to. We were hit by a Volvo a few years ago, guy started his petrol strimmer horse steps back to look, Volvo coming up the road , I signal to slow down give us a second to compose, he instead boots it straight at us, I kick the horse forwards, volvo just hits his back end knocking us sideways, luckily horse and me are ok, a real t**t . So very small brain with mixed logic in something with no brakes and extremely heavy so anything can happen, so my advice pass carefully :D