... Neptunus is often used metaphorically by the poets, to signify sea water.  In the Consualia of the Romans, horses were led through the streets finely equipped and crowned with garlands, as the god, in whose honour the festivals were instituted, had produced the horse, an animal so beneficial for the use of mankind. (Bibliotheca Classica: or, A Classical Dictionary, Lemprière, 1852)

... The name is derived by the Stoic Balbus, in Cicero, from nando (N. D., 2, 26), an etymology which Cotta subsequently ridicules.  (N. D., 3, 24.)   Varro deduces it from nuptu, because this god "covers" (obnubit) the earth with the sea.  (L. L., 4, 10.)  This latter derivation, though approved of by Vossius (Etymol., s.v. nuptæ), is no better than the former.  We may compare the form of the word Nept-unus or Nept-umnus with Port-umnus, Vert-umnus, and the word al-umnus; but the meaning or origin of the root Nept or Nep seems uncertain.  It may, perhaps, be connected with the same root that is contained in the Greek νίπτ-ω, "to wet."  (Keightley's Mythology, p. 85, seqq.—Encycl. Us. Knowl., vol. 16, p. 146.). (A Classical Dictionary, Anthon, 1842)