Beware of the Dog

Beware of the dog (also rendered as Beware of dog) is a warning sign indicating that a dangerous dog is within.

Warning signs of this sort have been found in ancient Roman buildings such as the House of the Tragic Poet in Pompeii, which contains a mosaic with the caption cave canem. Some suppose that these warnings may sometimes have been intended to prevent visitors from stepping upon small, delicate dogs of the Italian Greyhound type.

Under English law, placing such a sign does not relieve the owner of responsibility for any harm which may come to people attacked by the dog.

Such signs may be placed to deter burglary even if there is no dog.

...
Wikipedia