1. A dark horse
We never expected that he would be the champion in the race. He was such a dark horse!
2. Closing the stable door after the horse has bolted
Introducing tighter security measures after the break in seems to me like closing the stable door after the horse has bolted.
3. A horse of another color/a horse of a different color
Taking the exam is one thing but passing it is a horse of a different color.
4. Beat/flog a dead horse
Do you think It's worth me writing to a few more recruitment agencies, or am I just beating a dead horse?
5. Get off your high horse
If you would get off your high horse for one minute and listen to your colleagues, you would find that they have some great ideas for this project.
6. Don't put the cart before the horse
There's no point trying to write the report when you haven't got a clear idea of what to write. You don't want to put the cart before the horse.
7. To look a gift horse in the mouth (usually with a negative)
I know the car's not in great condition, but you shouldn't look a gift horse in the mouth.
8. Drive a coach and horses through
The clever lawyer drove a coach and horses through the criminal's excuses.
9. straight from the horse’s mouth
Lisa will be promoted to manager. The boss told me himself. It's straight from the horse's mouth!
10. To back the wrong horse
When the final result of the presidential election came out, many people found that they had backed the wrong horse.
11. Horses for courses
Ah well, it's horses for courses. Just because he is an IT expert doesn't mean that he is an expert in everything to do with computers.
12. hold one's horse
Hold your horses. Don't leave without finishing your report first.
13. horse around
My friends and I would horse around and try to push each other.