Donkeys were brought to Britain by the Romans who used them singly as pack animals, or harnessed up to four abreast to pull wagons and carts.
Donkeys can live a long time. Some as long as 60 years, but 40 is considered quite elderly.
This may be why we say 'donkey's years' as being a long time, but it might also be a play on words as they also have long ears.
'Donkey work' was always demanding and tough. But it is unclear why we say 'talk the hind leg off a donkey'!