This mosaic from Pompeii depicts a dog showing his teeth with 'beware of dog' spelled out underneath, but the dog is clearly in a play bow. The artist was careful to make this clear, even showing the ground(?) under the front legs so the viewer can see the dog's position. It says to beware of a dog who is showing that he wants to play.
This mosaic is different from other Roman depictions of guard dogs or hunting dogs. And, of course, even today we think the juxtaposition of a 'beware of dog' sign with a playful dog very funny.
Maybe!
We see the same design in a fresco described in Petronius' Satyricon:
As I was gaping at all this, I almost fell over backwards and broke a leg. There, on the left as one entered, not far from the porter's cubbyhole, 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
My colleagues laughed at me, but when I got my breath back I went on to examine the whole wall.
(Translation from Richmond.edu)
Certainly in this case, in a literary sense, it has been used for comic effect. Can we infer that that was the intention? Possibly.
However, in both the Trimalchio's house in the Satyricon, and the House of the Tragic Poet, the CAVE CANEM is prominent at the entrance to the house, which suggests at least the ostensible purpose of a warning to would-be burglars. Those two functions aren't mutually exclusive of course!
