Bran, warm water, and a little bit of grain :)
Here's the recipe i am used to do : place one pound of dry bran in a bucket, pour boiling water and stir to moisten.
You can chose the texture of the preparation depending on your horse's taste. Cover the bucket with a towel and let the bran steam for at least fifteen minutes. After that you can add carrots, apples, oats, molasses.
Serve warm to your horse :)