Here are some common English idioms about animals.
cat's whiskers = to think you are the best: "He thinks he's the cat's whiskers!"
like the cat that's got the cream = look very pleased with yourself: "He looks like the cat that's got the cream!"
cat got your tongue? = a question we ask when we think someone is guilty of something: "Why don't you say something? Cat got your tongue?"
let the cat out of the bag = tell a secret: "He shouldn't have told her about the party - he's let the cat out of the bag now."
put the cat among the pigeons = cause trouble: "Don't tell her about your promotion - that will really put the cat among the pigeons."
have kittens = panic: "The way he was driving, I was having kittens."
the bee's knees = think you're the best: "He thinks he's the bee's knees."
have a bee in your bonnet = be obsessed by something: "He's got a real bee in his bonnet about buying a new car. "
from the horse's mouth = get information from the original source: "I know they're getting married - I got it from the horse's mouth."
a white elephant = something that is expensive, but has no use: "People say the stadium is a white elephant and a waste of money."
a memory like an elephant = have an excellent memory: "She won't forget, you know. She has a memory like an elephant."
play piggy in the middle = be caught between two sides of an argument: "Because they aren't talking, I've been playing piggy in the middle."
make a pig's ear of something = make a complete mess of something: "You've made a right pig's ear of this. Let me do it!"
in hog heaven = be very happy: "We gave him flying lessons for his birthday - he was in hog heaven!"
pigs might fly! = something is as unlikely as pigs being able to fly: "Do you think the government will cut taxes?" "Pigs might fly!"
have butterflies in your stomach = be very nervous about something: "She's got butterflies in her stomach - it's her driving test today."
til the cows come home = do something for ever: "I can tell him til the cows come home not to be late, but he never listens."
take the bull by the horns = face a problem and take action: "I'm going to take the bull by the horns and tell him I've changed my mind."
get someone's goat = annoy someone: "It really gets my goat when she criticises him - it's not as if she's perfect herself."
a loan-shark = someone who lends money at high interest rates: "Don't borrow money from him - he's a complete loan-shark."
have a whale of a time = really enjoy yourself: "They went out and had a whale of a time."
like a fish out of water = feel very uncomfortable in a particular situation: "He feels like a fish out of water in a suit - he much prefers wearing jeans."
with your tail between your legs = feel guilty or ashamed: "He told us all that he was leaving, then he came back ten minutes later with his tail between his legs."
in the dog-house = when you know that someone is angry with you: "I'm in the dog-house - I forgot to do the shopping."
the lion's share = most of something: "She did the lion's share of the housework."
in the lion's den = in a dangerous place: "The interview was like going into the lion's den - they asked some very difficult questions."
a snake in the grass = someone who can't be trusted: "Don't tell him any secrets - he's a snake in the grass."
bug someone = irritate someone: "He's really bugging me about the holiday! I wish he'd just go away and leave me alone."
worm your way in = be nice to people so that gradually you get yourself into a good position with them: "He wormed his way into the finance department to get a job."
monkey about = play and not work: "Stop monkeying about, will you? We've got loads of work to do!"
make a mountain out of a molehill = make a big issue out of something small: "Don't worry about it - it's not important at all. You're making a mountain out of a molehill."