The Ass in the Lion's Skin

Aesop's Fables

AN ASS, having put on the Lion's skin, roamed about in the forest and amused himself by frightening all the foolish animals he met in his wanderings. At last coming upon a Fox, he tried to frighten him also, but the Fox no sooner heard the sound of his voice than he exclaimed, "I might possibly have been frightened myself, if I had not heard your bray."

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'.