according to Ovid ( Metamorphoses I) a son of Faunus and Symaethis. Other sources mention him as a son of Dionysus.
He was beloved by the nymph Galatea, and Polyphemus the Cyclop, jealous of him, crushed him under a
huge rock. His blood gushing forth from under the rock was
changed by the nymph into the river Acis or Acinius at the foot
of mount Aetna. This story does not occur any where else, and is
perhaps no more than a happy fiction suggested by the manner in
which the little river springs forth from under a rock.