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.
As far as we know Pluton seems to be an enormous battleship built by Water Seven carpenters in order to keep the balance with the other two Ancient Weapons powers.
Poseidon was the name of a mermaid who has the power to communicate with and control Sea Kings. After she died, her descendants who possessed the same ability to speak with Sea Kings were granted her name as a Title, known colloquially as the “Mermaid Princess”. Currently, this ability has been inherited by Shirahoshi, so Shirahoshi is Poseidon, one of the three Ancient Weapons.
Uranus: this is the most unknown Ancient Weapon. We only know its name when Robin named it, and that’s everything we have, nothing more. Since the fact that it is completely unknown, I believe that it’ll be shown us by a character who’s still alive and who’s as mysterious as this Ancient Weapon. Who could it be? In my opinion, that’s Momonosuke.
Why not Luffy, why not Roger, why not his own father- Kozuki Oden? Why just Momonosuke? Because he was the only one able to converse with Zou.
Now imagine someone with the ability to converse with all the mythical beasts of the One Piece world and order them at his/her command- wouldn’t this power/ability fit the tag of something like Uranus?- deemed to cause destruction of the world.
It may seem as a lame power for an ancient weapon, but as “Poseidon”, I don’t see Shirahoshi any different from Momonosuke. Shirahoshi herself is a mermaid princess who has the power to communicate with Sea Kings. But I don’t think that’s all of her abilities. I believe Shirahoshi can control Sea Kings to create tsunamis, outrageous tidal waves, and such puts her in the tier list of being an ancient weapon.
In comparison, in the same way, Momonosuke’s present individual strength should not be taken into consideration to decline the possibility of him being Uranus. His ability is not just converse, but command and seeing through the eyes of the very beast. It makes him a well candidate for being an ancient weapon.