THE FOX, THE DONKEY AND THE LION SKIN

A donkey put on the skin of a lion and went around frightening all the animals. The donkey saw a fox and tried to frighten her too, but she had heard his voice first, so she said to the donkey, 'You can be sure that I too would have been afraid, if I had not already heard the sound of your bray.'
Likewise, there are certain ignorant people whose outward affectations give them an air of importance, but their true identity comes out as soon as they open their big mouths.

In Perry 188, a donkey puts on a lion's skin and scares the other animals, but the fox realizes that the creature still brays like a donkey. In Perry 358, the donkey in a lion skin is able to scare the farmers so that he can graze freely, but the wind blows the skin off, so that the farmers realize their mistake and punish the donkey (or, as in Avianus, the man grabs the "lion" by his donkey ears; in Odo, the farmers first recognize the donkey's braying, and then see his donkey tails and hooves).