Idioms (animals)

It's raining cats and dogs! = It's raining heavily!
Ex: Wow! It's raining cats and dogs today and I forgot my umbrella.
 
To have a cow! = To get really upset
Ex: When I told him I would arrive home around midnight he had a cow!!

It's a dog-eat-dog world = Only the strongest survives
Ex: I've never learned how to use a computer, so I lost my job to a new employee. It's a dog-eat-dog world!

Hold your horses!! = Wait, stop!!
Mary: Hurry up!! We're going to be late!!
Sandy: All right!! Hold your horses, I'm coming!
 
To put the cart before the horse = to do things in the wrong order
Sue started driving some months ago, but she will take her driver's license only next week. She has put the cart before the horse!