What’s the Legendary Devil Fruit the Five Elders are referring to in Chapter 1037? It’s simply Uranus. It ticks all the boxes and I’ll explain why in the most straightforward way that I can.
1) The CP0 and the mysterious devil fruit Vegapunk was trying to create
The masked agent was thinking that it was convenient that the experiment of recreating the devil fruit was a failure.
As far as we know, the Cipher Pol have been trying to get their hands on the Ancient Weapons. Spandam tried to acquire the Pluton and we’ve seen the chaos of that in Enies Lobby, and we’ve been told explicitly they were trying to get their hands on the Pluton, so it’s not far-fetched to think they were trying to acquire other Ancient Weapons.
2) The hidden name of the Devil Fruit that has been lost in the Annals of history is Uranus.
It’s putting two and two together. The Ancient Weapons are of mass destruction, and unlike the Pluton which is a ship, a live weapon would be much more chaotic and unpredictable to control. The fruit was said to be lost in the annals of history and as far as we know the only person who knows about Uranus is Robin. Why? Because she read it on the Poneglyphs in Fishman Island. She is the only one capable of knowing the true name of the Ancient Weapons.
King Neptune indicated that the Government doesn’t know about Poseidon, so we can assume they have a vague understanding of Uranus and its name as well.
The reason the Government would give it a fake name is because its real name is only known to those who can read Poneglyphs.
Why would the name be lost in history? Because it is an Ancient Weapon and only the Poneglyphs, which no one but Robin knows how to read, knows the true name.
3) Momonosuke and the Actual Devil Fruit
Momonosuke’s devil fruit is Kaido’s devil fruit. Actually, let me be more precise, it’s Momonosuke’s Devil Fruit. That is why Vegapunk attempted to recreate it. It can’t be awakened except if Kozuki Momonuske specifically is the one that is the user. That’s why it was deemed a failure by the Cipher Pol, it’s because the vessel (Momonosuke) needed to actually bring forth its true power, its Awakening, wasn’t there..until the vessel ate it in Punk Hazard.