Short answers, pulled from the story.
Ancient grammarian Varro derived the Latin term from nuptus meaning covering or opertio. Modern scholars have proposed alternative theories including derivation from the Indo-European root neptu- for moist substance. The name may also come from the Etruscan deity Nethuns who took his name from the toponym Nepete near Falerii.
Neptune began as a Roman god of freshwater rather than the open sea before 399 BC when the lectisternium introduced Greek figures into Rome. By 399 BC Neptune absorbed the identity of Poseidon during this syncretic process and became identified as the brother of Jupiter and Pluto. Ancient sources indicate he supplanted Portunus as the god of naval victories by the first century BC.
The festival of Neptunalia takes place on July 23 at the height of summer. This date coincided with the period when water was scarcest in the Roman calendar. The most ancient Roman calendar set the feriae of Neptunus on this specific day between the Lucaria festival of the grove and the Furrinalia festival of July 25.
Salacia and Venilia functioned as paredrae accompanying Neptune representing fundamental aspects of his power over water. Varro connected Salacia to salum meaning sea while linking Venilia to ventus for wind. Augustine examined these figures in De Civitate Dei VII 22 and noted inconsistencies in defining their theological roles.
The Etruscan name for Neptune is Nethuns and his name appears on the Liver of Piacenza in section seven and section twenty-eight. The Liber Linteus contains eight instances of the name Nethuns across columns seven nine and eleven. A mirror from Tuscania depicts Nethuns talking to Uśil and Thesan holding a double-ended trident.