Nebulous Neptune

 Neptune Calming the Tempest  (Rubens, 1635)

I suppose it's fitting that the origins of the god Neptune's astrological connection to the seas is a wee bit nebulous. Astrological Neptune is known for nebulousness. Custom and practice does seem to have made it a relevant connection - sometimes.

While reading something else on Roman festivals in general I noticed this from The Roman Festivals of the Period of the Republic by William Warde Fowler at Google books HERE. Clicking on the segments will bring up clearer images.

And from crystalinks.com HERE:

Neptune is the god of the sea in Roman mythology, a brother of Jupiter and Pluto. He is analogous but not identical to the god Poseidon of Greek mythology. The Roman conception of Neptune owed a great deal to the Etruscan god Nethuns. Originally he was an Italic god paired with Salacia, possibly the goddess of the salt water. At an early date (399 BC) he was identified with Poseidon, when the Sibylline books ordered a lectisternium in his honour (Livy v. 13).

In earlier times it was the god Portunes or Fortunus who was thanked for naval victories, but Neptune supplanted him in this role by at least the first century BC, when Sextus Pompeius called himself "son of Neptune".

Neptune was associated as well with fresh water, as opposed to Oceanus, god of the world-ocean.

Like Poseidon, Neptune was also worshipped by the Romans as a god of horses, under the name Neptune Equester, patron of horse-racing.

Neptune was also considered the legendary progenitor god of a Latin stock, the Faliscans - ancient Italian people - who called themselves Neptunia proles. In this respect he was the equivalent of Mars, Janus, Saturn and even Jupiter among Latin tribes.

  Neptune Sending a Deluge to Troy, by Antonio Tempesta (17th century).

And there's this from tribunesandtriumphs.org:

.....In searching for the mythological sense of the fable, we must again have recourse to Egypt, that kingdom which, above all others, has furnished the most ample harvest for the reaper of mysteries. The Egyptians, to denote navigation, and the return of the Phoenician fleet, which annually visited their coast, used the figure of an Osiris borne on a winged horse, and holding a three-forked spear, or harpoon. To this image they gave the name of Poseidon, or Neptune, which, as the Greeks and Romans afterwards adopted, sufficiently proves this deity had his birth here. Thus the maritime Osiris of the Egyptians became a new deity with those who knew not the meaning of the symbol.

So, Roman Neptune took on characteristics of Greek Poseidon, Greek Poseidon took on characteristics of an Egyptian depiction. What happened in astrology, though, to Neptune's link with horses and "stock" - why weren't these attached to Neptune, as well as a connection to the sea?