My Thoroughbred Hank could have a semi-trailer drive past him without so much as flinching and then be convinced that a small piece of carpet on the ground was a horse eating monster for sure!!! He is now happily retired due to a dodgy me. Still miss riding him though.
My mare used to spook at a specific species of flower that grew on our hacking route. The flowers were lightish blue and I had to make sure we were only walking as we passed as cantering past led to some painful sudden stops (pommels are HARD!) No other flower bothered her in the slightest.