This one always confused me, why would a mouse bite a fox? Do foxes eat mice? It turns out they do, but they are known for preferring easy pickings – scavenging through rubbish, snatching chickens from farm yards. Mice and rats are not easy to catch, they’d be way down the bottom of the fox menu, surely.
Wouldn’t it make more sense to be a cat?
Yep, turns out 狸 here means cat.
窮鼠嚙狸（穷啮）qióng shǔ niè lí Lit: A desperate rat will bite the cat Fig: The downtrodden unexpectedly starts resisting or expressing anger at their situation
The Three Kingdoms encyclopedia Guangya (广雅，’Further Expositions’) defines 狸 as ‘cat’: “狸，猫也。”
The great Taoist sage Zhuangzi is quoted as: “捕鼠不如狸狌” ‘better to catch a cat or a weasel (狌 shēng, another version of 鼪, weasel or stoat) than a mouse.’
EDIT: Really important... I’ve come across this expression (穷鼠啮狸) in writing but never in spoken Chinese. So I’ve asked around and no native Chinese speaker I mentioned it to has heard of it. Instead, if you want to use an expression to convey this idea (’the worm turns,’ ‘the meek fight back’ etc) best to say:
tù zi bèi bī jí le yě huì yǎo rén
兔子被逼急了也会咬人
A loose translation of which would be:
Even the rabbit, if desperate enough, will bite back.
Or...
If its trapped, even a rabbit will bite.
This is a colloquialism, not a formal proverb.
