Horses normally take notice of anything unfamiliar to them and startle to some degree. They will then do one of two things: Either calm down fairly quickly and learn that the 'thing' is not important and is safe to ignore. Or instead there will be a continued startle response that is remembered each time the horse re-encounters the 'thing'.

The first is called habituation, the second is sensitisation.

Importantly habituation is highly context specific, for example a horse can get used to a specific plastic bag in a specific location on the yard, but will still startle when encountering a different one on a hack.

Sensitisation can occur without the horse ever being harmed by the unfamiliar, and anxiety, fear of the unknown can provoke dangerous defensive behaviour just as easily as actually being hurt by something.