An expensive mistake
a white elephant
When only one person can win
one horse race
Somebody you didn't expect to win doing well
a dark horse
Unable to speak
A frog in your throat
To take on a big challenge
take the bull by the horns
Choosing a different option when you've already started
change horses in midstream
When trying to find the reason something happened is dangerous
curiosity killed the cat
Looks scary, but not actually powerful
paper tiger
Fake crying
crocodile tears
Everyone wins sometimes
every dog has his day
Hearing something from the person who knows the truth
straight from the horse's mouth
Getting the exact write answer
bullseye