Ill say this now. Not my horse and the Vet said its nothing to worry about so they are no help. (or they don't care and second opinions are not affordable.)
My friend's TB mare has a head shaking issue. She dose it CONSTANTLY. Lol her neck is as hard as a rock XD. She dose it more when she is 'stressed' (or is unhappy about something and is having a 'moment') I have some vid, not the best angle though.
When she shakes her head she tightens her lips too.
I was wondering if anyone has seen this before and can give me more information. All I can find is allergies but she dose this all year round.