"Cave Canem"

The graphic design looks quite modern, but... "This mosaic of a guard dog, one of several in Pompeii, is on the floor of the entrance hall to the House of the Tragic Poet, which also is known for its fresco of the Sacrifice of Iphigenia."  Here's a contemporary citation:

"There on the left as one entered...was a huge dog with a chain round its neck. It was painted on the wall and over it, in big capitals, was written: Beware of the Dog."

Petronius, Satyricon (XXIX)

(The Petronius quote may be describing a different work.)

Found in the Encyclopedia Romana, via The Ancient World.