Ok, friends, here it is. I discovered this in Liz Graves' blog while researching The Smart Woman's Guide to Midlife Horses a few years back. Sadly, nothing stays cold enough for long enough here in Texas for this to work for my horses, but those northern sistahs who've tried it say it's a hoot!
Take a regular ol' 5-gallon plastic water bucket bucket and put a few inches of water in it. Leave it outside until it freezes. Now add some chopped carrots and apples and enough water to cover. Set it outside again and let it freeze. Add a little alfalfa or oats or other treat your horse loves, cover with water, let freeze. Layer this, lasagna style, till the bucket is full and frozen solid. (For ideas of fruits and veggies you may not realize are excellent horse treats — and which are NOT, check out my other Midlife contributor Dr. Juliette Getty's book, Feed Your Horse Like A Horse, the best book on Equine Nutrition I've ever run across.
Dip the bucket in a bathtub of hot water to loosen and then turn it out in your horse's pen. or stall for a toy and a treat that will keep him busy and likely provide you with hours of shareable Facebook video! (Do include me @MelindaFolse)