According to Professor Wikipedia, a canter is generally a speed of 10 to 17 mph. Therefore, I suppose if we are being really pedantic about it, no horse ever wins in a canter.
https://en.wikipedia.org/wiki/Canter_and_gallop
If the word is merely being used as a slang figure of speech within racing, what would constitute a canter? The jockey not moving a muscle? Not having to shake the reins? Not having to use the whip?