Imu is probably an Umibozu, a Giant Sea Yokai
It's said to be completely Black & combination of Dog & Sea Serpent, with it's Lower Half being a complete Mystery
They sink Ships, Storms appear when They show up & their names mean "Sea Priest"
In some Legends, it's said that it appears to Sailors & ask "Am i Fearsome", and when Sailors say "There is nothing as Fearsome as navigating through the Whole World", the Umibozu disappears
It's also said that They sink Ships by demanding Barrels from them, to which they use to scoop up Water & dump it into Ship's Deck, so it's like They use their own Offering against them
At Start of Thriller Bark Arc (Same Arc where Umibozo were Teased at the End of), We were Introduced to Concept of Sailors putting Offerings in Barrels to Sea God for Safe Passage, so it's possible that Umibozo in One Piece collect such Offerings/Barrels
Hence the Name "Sea Priests", it's like They are Priests of Sea God, and Oda already showed this Concept with Big Mom who created Completely Black Humanoid Incarnations of her will whose Job was to collect her Soul Tax, and They did it inside Churches if i remember right, it doesn't get more obvious than this
The only way to Escape these Umibozo is to give them a Bottomless Barrel, or in One Piece Version, an "Endless Offering", so that the Sailors can Escape while the Umibozo is confused & stuck trying to fill such Endless Barrel
Someone in One Piece represents The Offering, Someone else represents the Umibozo who wants to Collect such Offering for the sake of Sea God, and another Character represents the Sailor who escaped the eyes of Umibozo, meaning He opposes Sea God & didn't Pay
The Offering is "Luffy" who literally started his Journey inside a Closed Barrel
The Umibozo tasked to Collect such "Endless Offering" is none other than "Imu", The High Priestess of Sea God
The Sailor who opposes God & made this Offering is none other than "Man With Burn Scar", who Summoned Whirlpool in Chapter 2
We already know that Imu is a Human who has Powers to Transform into a Monster
We already know that Luffy is a Human who has Powers to Transform into Sun God
So Man With Burn Scar is also a Character who acquired Powers of an Ancient Sailor
And who opposed Sea God? None other than Vander Decken The First