I know that a lot of knitters knit coats for their dogs, but that's a bit unambitious. Think big - knit for a horse!
Particularly beneficial for nervous or excitable animals." The pattern has the usual warning that when you are buying cotton, you should buy enough to complete the article, as dye-lots vary. Obviously, it is important to avoid a stripe of a slightly different shade at one end of the cooler, especially if your horse is excitable.
And if you do have any yarn left over, you can make yourself a health vest.