I found this story on Futility Closet's blog here. The original source was the British Veterinary Journal, March 1888.
"A correspondent of the Manchester Sporting Chronicle, thinking that his horse was short-sighted, had his eyes examined by an oculist, who certified that the horse had a No. 7 eye and required concave glasses. These were obtained and fitted on to the horse’s head. At first the horse was a little surprised, but rapidly showed signs of the keenest pleasure, and he now stands all the morning looking over the half-door of his stable with his spectacles on, gazing around him with an air of sedate enjoyment. When driven his manner is altogether changed from his former timidity; but if pastured without his spectacles on, he hangs about the gate whinnying in a plaintive minor key. If the spectacles are replaced he kicks up his heels and scampers up and down the pasture with delight."