Eager beaver
A person who is excited about doing certain work
Make a beeline
Go straight for something
Fishy
odd, suspicious
Let the cat out of the bag
Reveal a secret
Cat nap
A short sleep
Until the cows come home
For a very long time
Have a cow
Get extremely upset (often over something minor)
Pig out
Eat a lot of something
(a) little bird told me
I heard something (usually secretive or unknown) from someone (not named)
In the dog house
In trouble with another person
Copy cat
A person who does the same thing as someone else
Straight from the horse's mouth
Directly from the original source
Monkey see, monkey do
Silly/unintelligent people tend to copy each other's actions
Raining cats and dogs
Raining heavily
Smell a rat
Begin to suspect trickery etc
Hold your horses
Wait and be patient
Take the bull by the horns
Face a challenge or danger boldly
Chicken out
To decide not to do something out of fear (usually just before)
Cat's got one's tongue
Said about someone who doesn't speak (usually due to shyness)