I would absolutely not recommend using WD-40 on ANY animal or person! It's not a cleaner in the same line as a detergent, shampoo or soap. It is composed of mineral oils and light lubricating oils. It is a lubricant and fine oil spray, and is used for lubrication and cleaning water out of metal joints to prevent corrosion. It also dissolves cocaine, so spraying a loo seat with it in a club prevents people from snorting cocaine there.
I don't know much about horse grooming (not a horse owner!) but try horse-safe detergents and lots and lots and lots of water. NOT WD-40, or white spirit.
EDIT: I just checked the can we have in the garage, and while it doesn't carry a HARMFUL or IRRITANT symbol, it does say to avoid contact with skin and to not breathe the fumes, vapour or spray.
WD-40 is a good solvent for many greases and displaces water, which is why it's used to prevent corrosion. But you'd be better off keeping it for when the buckle on your girth siezes up or something.
If you believe everything you read, better not read.