Neptune appeared in 1974 TV series called Kamen Rider X. Neptune (ネプチューン Nepuchūn) is a Neptune-themed piranha monster who killed Jin's father.
Exclusively in Pachinko Kamen Rider: Full Throttle, he have an ability to transform into Dark X (闇のエックス Yami no Ekkusu), the dark version of Kamen Rider X, and the fifth of the Dark Shocker Army's Dark Seven Legendary Riders. He was destroyed by the X Kick.
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An aquatic cyborg, Neptune could function both in sea and on land. He possessed the ability to spew foam-like bubbles from his mouth that would cause instant death upon contact with humans, reducing them to skeletons. Befitting his motif, Neptune wielded a trident as his weapon.