museum-of-artifacts:

One of the oldest “Beware of the Dog” signs in the world. Domus del Poeta Tragico, PompeiiThe House of the Tragic Poet  is a typical 2nd century BC Roman house in Pompeii, Italy. The vestibule floor was decorated with a mosaic picture of a
domesticated dog leashed and chained to an arbitrary point. Below the
figure were the words “CAVE CANEM”, an ancient warning equivalent to the modern “Beware of the Dog”.