An ass. It was made to rhyme with “monkey,” but is never now so pronounced. The word means a little tawny or dun-coloured animal.
Donkey. The cross of the donkey's back is popularly attributed to the honour conferred on the beast by our Lord, who rode on an ass in “His triumphant entry” into Jerusalem on Palm Sunday. (See Christian Traditions.)
The donkey means one thing and the driver another. Different people see from different standpoints, their own interest in every case directing their judgment. The allusion is to a fable in Phædrus, where a donkey-driver exhorts his donkey to flee, as the enemy is at hand. The donkey asks if the enemy will load him with double pack-saddles. “No,” says the man. “Then,” replies the donkey, “what care I whether you are my master or someone else?”
To ride the black donkey. To be pigheaded, obstinate like a donkey. Black is added, not so much to designate the colour, as to express what is bad.
Two more, and up goes the donkey—i.e. two pennies more, and the donkey shall be balanced on the top of the pole or ladder. It is said to a braggart, and means—what you have said is wonderful, but if we admit it without gainsaying we shall soon be treated with something still more astounding.
Who ate the donkey? When the French were in their flight from Spain, after the battle of Vittoria, some stragglers entered a village and demanded rations. The villagers killed a donkey, and served it to their hated foes. Next day they continued their flight, and were waylaid by the villagers, who assaulted them most murderously, jeering them as they did so with the shout, “Who ate the donkey?”
Who stole the donkey? This was for many years a jeer against policemen. When the force was first established a donkey was stolen, but the police failed to discover the thief, and this failure gave rise to the laugh against them.
Who stole the donkey? Answer: “The man with the white hat.” It was said, in the middle of the nineteenth century, that white hats were made of the skins of donkeys, and that many donkeys were stolen and sold to hatters.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894