Appearances are deceptive.

Aesop (620?-560? BC) Legendary Greek storyteller

Fables [Aesopica], “The Wolf in Sheep Clothing” (6th C BC) [tr. Jacobs (1894)]
    (Source)

Alternately, "Appearances often are deceiving." Versified by Gaius Julius Phaedrus, Fables bk. 4, as "Things are not always what they seem."

Note that there are two fables by this name. In this one, a wolf prospers by wearing a sheepskin he finds and drawing other sheep away to be eaten. In other versions, the wolf sneaks into the sheepfold wearing the skin, and then is killed and eaten by the farmer who wants sheep for dinner.