Neptunus

Japanese: Neptunus

…an ancient Roman god who protected stored grain. He was identified with the horse-god Neptune, because chariot races were held during his festivals, and was also worshipped as the god of councils, in analogy with the consilium (consultation). His altar was located underground in the Circus, and was only excavated on festival days, August 21 and December 15, which marked the end of the harvest and autumn sowing, respectively.

From Poseidon

…He was also the god of earthquakes and horses. He corresponds to Neptune in Roman mythology. He was the son of Cronus and Rhea. …

*Some of the terminology that mentions "Neptunus" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…貯蔵された穀物を守るローマの古い神。その祭りに戦車競走が行われたため，馬の神でもあるネプトゥヌスNeptunusと同一視されたほか，コンシリウムconsilium(協議)との類推から，会議の神としてもあがめられた。祭壇は大競技場の地中にあり，それぞれ収穫と秋の播種の終了期にあたる8月21日と12月15日の祭りの日だけ，掘り出された。…