The phrase "wrong horse" means when someone chooses or supports the incorrect person, idea, or outcome. It refers to making a mistake or picking the losing side. Full definition
I guess YOU»RE not as SMART as YOU think BeckyJL — the «message» you took from this isn't what you say it is — put on your regular people had and get off your narcissistic horse — read it again — IN TOTAL this time... YOU got it wrong (drkellyflanagan.com)
Life on the political inside comes with a certain amount of risk, and in his long career Kinsella has been accused more than once of giving bad advice or backing the wrong horse. (thetyee.ca)