Poseidon is the name of a sea-god in classical Greek mythology, later known to the Romans as Neptune. It was mentioned by H. P. Lovecraft in his 1926 short story "The Strange High House in the Mist."
In the Mythos
In Lovecraft's Works
Father Neptune once appeared to Thomas Olney, holding a trident and accompanied by scores of tritons and nereids, as well as the primal god Nodens. (HPL: "The Strange High House in the Mist")
Expanded Mythos Works
According to David Parker, Neptune, Dagon and Cthulhu are all different names for the same entity. (EXP: "Dagon's Bell").
In the expanded Cthulhu Mythos, Poseidon is a powerful extragalactic entity, awed by 'Ymnar. It battled against the Elder God Paighon.(source?)