You might also want to check this horse’s magnesium level, as this sounds to me a lot like a magnesium deficiency. I finally remembered to look up the web site address for the company from which I bought my magnesium supplement:
Not all magnesium supplements are the same, and which one you use can make a major difference. This company has (or had when I last checked out the website) a list of typical symptoms of magnesium deficiency, and what amazed me was how many negative behaviors can be a result of an undiagnosed magnesium deficiency. Sometimes it is less expensive to put the horse on a magnesium supplement rather than testing for it first, and if one follows the direction, the extra magnesium will not harm the horse, if a magnesium deficiency it not the problem.