I have a "special" gelding, would be much more difficult to find something he wasn't terrified of.
Yesterday, it was his new water trough.... yup, a big ol' scary, horse-eating water trough. Spent a good half hour, blowing, snorting and running in circles, then stood as far back as possible and giraffe necked to see what exactly was in it and slowly progressed to tasting the water and finally just decided to start chewing it up.
I had a trail horse that spooked at absolutely nothing. You just couldn't spook this horse until one day she decided the yellow concrete post marking the end of the irrigation ditch was SCARY! The same yellow post we rode past every.single.day!