Petronius, Satiricon, 29 :
Ad sinistram enim intrantibus non longe ab ostiarii cella canis ingens, catena vinctus, in pariete erat pictus superque quadrata littera scriptum
"On the left side when you enter the house, not far from porter's lodge, there was a huge chained up dog painted on the wall and above there was an inscription in capital letters reading CAVE CANEM - Beware of the dog !"
Such a chained up dog was represented on a mosaic at the entry of the Casa di P. Paquio Proculo at Pompeii (Insula 9, n° 1), but without inscription.
The moulding of a once real chained up dog that died in the eruption of the Vesuvius can be seen at the Antiquarium of Pompeii.