My horse has been having what looks like muscle spasms. He shakes his shoulder non stop for a long time and I don’t think he realizes it, even when there are no bugs. What can it be and what can I do?

I agree with @caoimhesweeney also if he was wet from rain or if it is cold at all then that might be it. It was raining the other day and a bunch of horses got wet and cold so they were shaking (the spot I always look at is the shoulder).
Get a vet out. It may be that he has a mineral imbalance, it may be neurological, it may be something else entirely.
Join the fun and sign up to connect with our 200,000 members!