I'm so sorry this happened. Prayers for Kenzie. I have never used Underwoods but I've heard great things about it, and it's used on major injuries. Here
is the link to their website.
Also, I wouldn't assume the note was meant for you. I could have been meant for someone else that works around the barn, or the BO, or really anybody. Someone twisted enough to do something like that for revenge or as a "warning" is obviously not a rational thinker, nor would they likely care if they had "the right horse" or not.