Originally Posted by Ahsisi
I would sponge bathe him on that side if it is frightening him. Each time try to go a little further up with the hose, but then stop if he gets worried and use the sponge. He should eventually let you hose the whole side. Some horses are just funny about water on one side compared to the other. Another thought is that maybe it has to do with the sound of the water on that side. If he doesn't mind you sticking your fingers in his ear, you could try an earplug and see if that helps at all. Good luck!
I agree with Ahsisi. Like all training, and this is training especially if the horse has any worries about what is happening, do the approach and retreat method. Yes you probably won't be able to use the hose to get him wet by the ear, but eventually you will as you continue to work with him with the approach and retreat method. Use the sponge to finish the job, and the hose to "train" with in that area.
Also, one of my horses is extremely one sided, and working him on his right side 90% of the time has greatly helped him. He was also very worried about the hose (big horse eating snake spitting poisonous liquid at him), and the approach and retreat helped greatly. When I retreated, I simply went back to an area, like his shoulder , to reward him and take the pressure off that he felt when hosing upper neck.
Hope I made sense. Good luck.