The House of the Dog Monnus, situated to the north-west of the Roman Gate, was originally a republican building, with a portico of tufa piers. It was rebuilt in the Trajanic period (opus latericium). There are many later modifications. The function of the building is not clear.

In the north part is a black-and-white mosaic with marine animals and monsters, and a Nereid. Below the Nereid is a dog wagging its tail. An inscription in the mosaic gives us the name of the dog: Monnus ("Darling"?). The mosaic has been dated to the early third century.