OH did a treatise on this for her HND having noticed that her Selle Francais x TB was petrified with fear by half a dozen small pigs near the yard. And 650kg of petrified takes some handling.
She did a series of control experiments with a dozen horses on the yard to test for general spookiness. These involved things like opening umbrellas suddenly, making noises and so forth. Each horse was led on a lunge line and the distance of deviation from the straight path measured. Then she did some tests using combinations of piglike dummies, genuine pig poo and finally genuine pigs.
The horses could spot the genuine article and all but one made a large diversion around the pigs, larger than any other disturbing event. The exception, an 18hh fellow, was completely imperturbable. Nothing including the pigs moved him, but he was scared of people. Any new person entering his box would get th full ears back retreating to the far corner treatment.