Join Date: Mar 2012
Location: Whidbey Island Washington
Do you normally trot when headed home? If not, I'd say THAT'S your problem, not the rain. Most horses will get a bit jiggy if you let them hurry towards home.
Next question: Is he on turnout? Does he have shelter? If so, does he USE the shelter, without fail, every time, when it starts to rain? If the answer to the last two questions is 'no', then rain is not the problem, see my first observation.
Last question: Was there a lot of noise associated with the rain? If so, try leading him to a similar area when its pouring out so he can become accustomed to the noise of the rain on shrubs and trees.