I can't say I've ever seen this particular behavior before. This is not the first…
As the sun set, something unseen and unheard triggered this large group of horses to move away from their quiet grazing on an open hillside. They filed up and into the woods, traveled single-file through brush and over a tree-lined hilltop, then disappeared into the coming night.
• an innate, typically fixed pattern of behavior in animals in response to certain stimuli
• a natural or intuitive way of acting or thinking