Spoilers say "Give another Name to it", so it indicates something with Two Names (Common One & Legendary One)
It could be Zunisha but i'm doubtful because Minks living on Top of a Millennial Elephant is something many people heard about, not to mention that Zunisha never abandoned it's Duty or disappeared, it has been Walking for 1000 Year.
So Gorosei questioning this Shadow, calling it a Legend that didn't awaken for long time, suggests that it's not Zunisha.
Here are all Possibilities:
1. Oni King DF (Basically Body of Onigashima) or Devil/Yokai Himself (Basically Originator of Devil Fruits)
2. Uranus (Whenever there are Cipher-Pol, there is High Chance we see Revos, so Dragon could have Unleashed Uranus)
3. The Creature from Egg in Roger's Ship (My Guess is that it should be a Bird)
4. The Transforming Robot from Sora Comic Story (Could it be that Queen was the one who Awakened it?! or Vegapunk is here?)
5. King of Elbaf is here (Basically he represents Odin & only awakens when The World Balance is endangered or when he sees a Prophecy)
6. Lurking Legend is finally here (He is a Wotan, that's all i can say for now)
7. A Third Azure Dragon (The Original One) ---> If this Happens, ZKK Theories gonna go Crazier
8. True Form of Zunisha (There is very High Chance it's indeed Zunisha, especially since Oda revealed Identity of Shadow right away)
9. Eye-Patch Legend is finally Introduced (All i can say is "Red")
... etc
Which one you guys hope it's True?