Minnette Rice Edwards answers:
This is difficult to answer without me seeing you move your mare sideways. However, my first thought is that she is particularly stiff on one side, so finds it difficult to cross her legs unless you give her enough room to do so.
When you are riding her, it is possible that you allow her more space as she moves laterally, but when you are on the ground, you block her progress. She may well have some deep scar tissue which would not normally affect her, but does just ‘catch’ her with this particular manoeuvre.
If she is trying to bite you and is normally good-natured, then it’s obvious she is trying to tell you something. We often forget to consider the horse’s neck and how it can affect the whole body. From the ground, you may be holding her head in a certain way which, due to some old injury, could be giving her a sharp pain in her neck or poll.
At the poll is the atlanto-occipital joint, where the skull attaches to the vertebral column, and this is a very delicate area. Humans have the same joint, but because we stand in the vertical and not the horizontal, our skull sits on the top of the neck, instead of hanging from it.
Just behind the ears are short, vulnerable muscles – these can easily be damaged, and become very painful and impacted if the horse is constantly ridden with a tight rein. They would certainly not take kindly to being twisted in your efforts to move her sideways.
The other part of the neck that may not show up in a normal vet check is the area around the withers. This relates to the seventh cervical vertebrae at the base of our own neck, which can become stiff and sore, particularly if we are tight in our shoulders.
Check it out
I suggest you try to move your mare sideways gently, giving her room to move forward at the same time. Be careful not to twist her neck, as the handler below is doing, and observe her to see at what point of the manoeuvre the discomfort affects her. And maybe your vet could recommend a physiotherapist who can pinpoint the problem.