There is also an early medieval version of this story, by a compiler of Aesop's Fables named Avianus. It is written in hexameter (that's poetry, folks), though I'll leave it in prose form:

Once an ass found a lion's skin, and putting it on began to romp around the pastures, frightening the herds. The farmer, hearing this, ran out, and taking the ass by its long ears said to him, 'You may be able to fool all the others, but I who know you will always recognise you for the ass that you are'.