This is a very old saying based on superstition and it appears that the phrase “hag-ridden” applies to both people and horses. A hag-ridden person is one afflicted by nightmares or anxieties. A typical sentence might read, “He was hag-ridden by his early success”. It means that he was worn out by worry.
In respect of horses, back in the days of a belief in female witches (people still believe they exist), when a stable man returned to the stables in the morning and found a horse in his charge drained of energy and bathed in sweat he might possibly conclude that his animal had been “hag-ridden”.
The phrase describes a belief that during the night an evil witch had entered the stables and stolen the horse. In attending witches’ meetings she would have ridden the horse all night to complete exhaustion whereby in the morning the animal was out of breath and covered in sweat.
by Robert Herrick, 1648
The hag is astride
This night for to ride,
The devil and she together ;
Through thick and through thin,
Now out and then in,
Though ne'er so foul be the weather. pic.twitter.com/lh2VFQOdrK
— Tony DiTerlizzi (@TonyDiTerlizzi) October 28, 2019
Perhaps the stablemen were equally convinced to the point where they went out and found a likely elderly lady to be a suspect at which time she would be persecuted as a witch. This would be incredibly sad because the reason why the horse might be in this concerning condition was because the stables in which it lived were highly unsuitable because they were designed for security without windows and without air. The stagnant air and a lack of oxygen would have caused the horse to be drained of energy and bathed in sweat by the morning. Another example of the thoughts of misguided humans attaching superstitions to horses.