a dark horse: a person who wins a race or competition although no one expected them to

Example: 19-year-old David Patrick could be a dark horse for a medal (= she could win) in the Olympics.

a Trojan horsesomeone or something that attacks the group or organization it belongs to

Example: Traditional Republican supporters have accused the new leadership of being a Trojan horse trying to destroy the party from within.

to back the wrong horse: to support someone or something that cannot win or succeed

Example: Fred backed the wrong horse and lost his bet.

to beat (or to flog) a dead horse: to waste time doing something that has already been attempted

Example: Do you think it’s worth sending my manuscript to other publishers or I am just beating a dead horse?

to put the cart before the horse: to switch around the proper order of things, to not do things in the right order

Example: Don’t put the cart before the horse, you need to finish your studies before you get married.

to get something straight from the horse’s mouth: to get information from a reliable source, from someone who knows for sure

Example: If you don’t believe me you can go get it straight from the horse’s mouth and Jake, he’s the one who saw it happen.

to get off your high horse: to stop acting as if you are better or more intelligent than other people, to be more humble

Example: Who do you think you are?! Get off your high horse and listen to what other people have to say, you might learn something.

to hold one’s horses: to slow down, to be patient

Example: Hold your horses, if you hurry too much you’ll make a mistake.