Donkeys do not live for donkeys years. Like the good, they die young. This is, I suppose, a terrible shame if you're a donkey. The phrase donkeys years therefore makes no sense at all until you remove the Y from years; because donkeys do have very long ears indeed.
Years and years and donkey's ears, as the saying is.
(Wright, Rustic Speech, 1913)
And that's how it was until somebody got confused and extended the life of the ass. If a donkey had lived for donkeys years, and if it weren't asinine (from ass), and if it were able write good English verse, and if it were the particular donkey upon which Jesus rode into Jerusalem, then it might have written the following:
When fishes flew and forests walked
And figs grew upon thorns,
Some moment when the moon was blood
Then surely I was born;
With monstrous head and sickening cry
And ears like errant wings,
The devil's walking parody
On all four-footed things.
The tattered outlaw of the earth,
Of ancient crooked will;
Starve, scourge, deride me: I am dumb,
I keep my secret still.
Fools! For I also had my hour;
One far fierce hour and sweet:
There was a shout about my ears,
And palms before my feet.
But it didn't: G.K. Chesterton did. Anyhow, I shall leave you with the sobering thought that a slang term for a straw hat is a donkey's breakfast.
Here is a harrowing song about cruelty to donkeys. What is worse is that the abuse described appears to have been perpetrated by a small child. Every time I hear this song, I think about the poor donkey and weep.