The Ancient Weapons are three weapons capable of mass destruction, each taking a different form. They are known as Pluton, Poseidon, and Uranus, each named after a deity for their godly powers. The weapons are not limited only to inanimate objects, but consist of living beings as well.
The Ancient Weapons are the argument used by the World Government to forbid research of the Void Century, since they fear that knowledge of the weapons could result in a global war.
While Pluton has yet to appear in the story, it has been referred to as an ancient ship capable of massive damage and destruction. It was built on the island of Water 7 sometime during the Void Century, and the shipwrights kept the blueprints as a safety measure. In the event that Pluton fell into the wrong hands, another one can be built to fight the existing one. Fortunately, the ship was lost to history before that could happen. The blueprints were passed down through the generations until they were passed to Franky. When Franky was captured by the World Government, he kept the blueprints hidden within his own body until he understood that Nico Robin, the only living person who could read the Poneglyphs and subsequently revive the Ancient Weapons, had no intention of doing so or even planned on using that power to destroy the world; he subsequently burned the blueprints. While the exact whereabouts of Pluton remain unknown, the Alabasta Poneglyph contains information on its location.
Poseidon was a Mermaid Princess who lived in the Void Century that had the power to talk to Sea Kings, enormous sea creatures capable of mass destruction. Princess Shirahoshi also has this power, and as stated in a conversation between Neptune and Robin, she has now inherited the title “Poseidon”.
Nothing is currently known about Uranus, except its name. I believe Uranus is a combination of Momonosuke and Zunisha. We know for a fact that Pluton is an inanimate object, a ship, and Poseidon is a human and i feel Uranus is an animal or human animal combo. Momonosuke’s speaking to Zunisha, Zunisha’s monologue when attacked by Jack stating that he was condemned to walk for eternity until the order was given for his return and Zunisha responding to Momonosuke’s voice alone is what the backbone of this theory. Momonosuke, Shirahoshi and Vivi all will play a crucial part in the future.
*Theory by kaizokugari