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 fox
someone 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 lie
to 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 mule
someone 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 horse
a person of unknown abilities or a person who has kept his abilities to himself and may surprise everybody
Who would have thought George would win the competition? He’s a real dark horse.
This is a racing metaphor, which says that an unknown horse which could win the race unexpectedly.
no room to swing a cat
a very small, cramped place
This room’s not big enough to swing a cat in.
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.
to put/set the cat among the pigeons
to provoke a quarrel
You shouldn’t have criticised the boss in your speech; now you’ve really put the cat among the pigeons.
One 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 kitten
feeble, very weak, having no strength
After her operation she felt as weak as a kitten.