Most Greek gods (as Athena) can only exists in singular, one exception being Hermes (Ἑρμῆς). So why Hermes? The answer is rather easy, the ancient Greeks had a special type of boarder markers called Herms (plural) - and thus a use for the god in plural.
A herm is recognized by the fact that it's a head on a short plain pillar with a (commonly erected) penis in the middle. Compare to this one.
"I have already stated that the Athenians are far more devoted to religion than other men. They were the first to surname Athena Ergane (Worker); they were the first to set up limbless Hermae, and the temple of their goddess is shared by the Spirit of Good men."
Pausanias - Description of Greece I.24.3
Translated by Jones, W. H. S.