Animals feature strongly in idioms. This is logical if we consider the various habits and characteristics of animals that we have lived side-by-side with for centuries.
a sly fox / to be as sly as a foxSomeone who is very experienced and has acquired a lot of guile.
You can’t trust him; he’s as sly as a fox.
to let sleeping dogs lieTo leave well alone and refrain from starting trouble.
You must have known that mentioning his ex-wife would upset him. You should have let sleeping dogs lie.
as stubborn as a muleSomeone who is unwilling to listen to reason or change his mind.
It’s a waste of time trying to get him to change his mind; he’s as stubborn as a mule.
a dark horseA person of unknown abilities or a person who has kept his abilities to himself and may surprise everybody. This is a racing metaphor which says that an unknown horse which could win the race unexpectedly.
Who would have thought George would win the competition? He’s a real dark horse.
no room to swing a catA very small, cramped place. The original phrase was probably ‘not room to swing a cat-o’nine-tails’, and dates from the time when sailors were flogged (whipped) on ships. The floggings took place on the deck because the cabins were too small.
This room’s not big enough to swing a cat in.
to put/set the cat among the pigeonsTo provoke a quarrel.
You shouldn’t have criticised the boss in your speech; now you’ve really put the cat among the pigeons.
a dog’s-bodyOne who does the routine or mechanical work, especially that which no one else wants to do.
When I worked in the factory I was the dog’s-body; I was given all the worst jobs.
as weak as a kittenFeeble, very weak, having no strength.
After her operation she felt as weak as a kitten.