There are many possible answers to your question, its not easy to address. Might be some type of feed deficiency. Go online and look up Springtime inc. I use several of their products and I have been pleased. Perhaps it is an immunity issue.
Hope this helps...

 Sooooooooooo sorry. I don't have a pony, but I read a lot about them, and not positive results. I think your horse is scared of a lot of things, but it may also be sick. I would take him to the pro-vets.
Hope your horse gets well!
Gog:}