Possible Clues:

• Part of bay for example dries up
• Part of equine anatomy that shrivels
• The part of a horse that droops?
• Highest part of horse's back
• Part of the horse droops?
• What gets dry when they're on the horse's back
• Languishes; part of horse near shoulder-blades
• Writhes on ridge between a horse's shoulder-blades
• What dries and dies on top of a horse
• Dries up on horses