The Reims Cernunnos, shown above, is one of the most famous images of a Horned God from antiquity. A product of Roman-era Gaul, it shows the Antlered seated cross-legged on a plinth or altar in a miniature temple. Apollo and Mercury attend him. In his lap, he holds a bag of circular objects—generally identified as either coins or grain—which he pours out to a bull and a stag before him. Mysteriously, in the pediment above him is carved a rat (or mouse; for the purposes of this discussion, the two are interchangeable).
There's much to be said concerning the symbolism of this relief, but here I will focus only on the most curious aspect of its iconography: the Rat.
The Horned Lord tends, in pantheon after pantheon, to be a Master of Animals generally, but still the choice seems an odd one. The scholarly literature has tended to address this question cursorily, generally preferring one of two readings.