luckylouie, it may help to remember there is no such thing as a horse that cannot be beaten. Every horse in a race has some % chances of winning. That also means every horse in a race has some % chances of losing.
When someone tells you a horse is "a certainty" or "it cannot lose" it pays to always take that with a grain of salt.
What I do is assess a horses chances of winning a race. I back horses where I obtain significantly better odds than my assessment of the horse's winning chances. I always take into account that every other horse has some chance, however small, of winning the race.