Bathing is something that you need to take time with your horse. Lots of horses start out not liking it, but gradually come to tolerate it, and eventually like it!!! Believe it or not, you will be able to do this with your mare.
Okay, here's my suggestions:
Pick a really hot day, like over 90. (In Idaho, that's easy!) Pick a day when the both of you can play in the water. Wear water proof sandals, or tennis shoes that can get wet, because they will! If you work her before hand, that's even better. Don't pick a time right before a show. Don't have a time limit...YOU need to be relaxed, and have the idea that this is a training session. You want her to have a chance to get used to this. Bathing shouldn't be JUST because of a show.
Tie her securely to something she can't pull up. I had a mare (a big one) who hated water. I put her in a vet's exam chute (I forget what you call them) and cross tied her in it. It's hard to find these things for minis. So secure yours to a tree, or a fence POST that she can't pull up. Cross tie her if possible.
Then I turned the hose on. Yes, it was cold water. I didn't turn the water on all the way, just a decent stream coming out of the hose. I don't use a spray nozzle, I just let the water run out of the hose. I start at the feet. If she moves around, don't repremand her, just keep the water on her. When she stops moving, praise her, and keep it up.
Gradually work the water up her leg, and onto her back. If she is like my horses, after a good work out, they appreciate having their legs and backs hosed. Don't hose her face, and in this first bath, don't hose too far up her neck. The idea here is to teach her that the hose won't eat her, and she won't melt in water! Hose her tummy, but if she gets too upset, back off a bit. Keep the water soft, and comfy...this should be a PLEASURE...she'll figure it out soon enough.
Use baby shampoo to wash her with...it won't hurt her eyes, and it's very gentle.
Then let her loose so that she can have a good roll. It will teach her that bathing doesn't necessarily mean a show. Half the pleasure of taking a bath (for a horse) is to get all muddy and dirty again!!!
The big mare I worked with was 15 at the time. I bathed her daily during the summer because I used her for a lesson horse, and she worked hard. She got to the point where she actually LIKED bathing. I would take the hose, and put it between her teats, and she would actually raise her leg so that I could get the dead skin from between them! I could hose her face, and she would drink from the hose.
I'm teaching my minis the same way. Bilbo didn't like baths very well, until I discovered that he hated being sprayed...but likes just a stream of water. He also likes to play with the hose and spraying me!
The idea is to PLAY...softly and gently. Have patience. Keep your horse and yourself safe. Go slow, and praise a lot. Before too long, you and she will discover that it's a good time to have together. This is often an issue of trust, not just whether or not they like cold water. Teach her that she can trust you enough to let you hose her down.
Kim R.