“horses have a unique ability to just accept their situation.” This is a quote from Bob Bailey. This being the reason they are so trainable. However we have a duty of care towards them to make sure we never abuse this trainability. I saw a video recently of a very shut down horse being described as an introvert – this apparently was why the horse was so quiet. To me the horse just looked depressed and was putting up with the things done to her. Please ditch the labels and learn to look at the whole horse, posture, facial expressions etc not just whether they are quiet and accepting. Many horses put up with what we do due to being punished for incorrect behaviour – they then become too worried about doing the wrong behaviour so they do nothing. Correction is a word often used as a euphemism for punishment.
This article has good images of depressed/shut down horses compared to alert and resting posture.