This mysterious monster is a pit left by Tolkien's grandfather, and there is not much information to find, and it only appears twice in the original book.
The first was after the Battle of the Five Armies, when Baring (A close friend of Thorin's) led a group of dwarves to try to retake Moria, but encountered a large number of orcs in the dungeon, and when they retreated to the Hall of Moria's West Gate to retreat, the monster attacked them and dragged Eun (one of the dwarves of the Lone Mountain Mission) into the water to kill. The dwarves who were blocked from their way had to return to Moria, and the remaining 40 or so dwarves were all killed in the Mazapur Palace.

Bahrain
Eun
Mazapur King Hall
The second time, as seen in the movie, the Ring Squad came to Moria's Gate. After Gandalf unlocks the secret of the door, the monster emerges from the water and grabs Frodo with its tentacles in an attempt to drag him into the water, the crowd joins forces to repel it, saving the ring bearer and retreating into the pit, while its huge tentacles come again, destroying the door beams of the pit and causing it to collapse, and the Guard Squad is forced into the depths of the pit to find a way out.
Moria Simon
The Guard Retreat retreats to the Mazapur Palace, and the Book of Mazapur discovered by Gandalf is written by ori ,son of another lonely mountain mission, which chronicles how the dwarves were attacked by monsters, whom they called watchers in the water.
The Book of Mazapur
Ori in The Hobbit and Ori in The Lord of the Rings
According to Gandalf's subsequent accounts of the crowd, he did not know much about the water watchers. Since it came up and went to Frodo, it can be speculated that it was Sauron's minions, and it was also attracted to the Supreme Ring.
We look through the history of the Durins who founded Moria and find no mention of this monster lurking in the water. It has been speculated that the water watcher came here after Moria was occupied by fire demons and orcs, and then went down the gray river from the ocean to here - the water watcher may be a sea monster (kraken).
Mirko had always wanted to touch the ocean after his fall, but the ocean had remained relatively pure under the management of ulmo (the lord of the sea in Vera). U-Omu never trusted Mirko, and his strength was very strong, no less than that of Manwei, the king of Vera, and Mirko could not do anything with him.
U-Omu
Just as the orcs were created for laughing at the elves, and the trolls were created for laughing at the treemen, this time the Dark Emperor created the sea monsters to mock the ocean of Uom.
However, because Mirko's focus has always been on Middle-earth, the sea monsters have rarely appeared after they have been created, and several major wars with elves and Vera have been mainly army and supported by the air force, so these sea monsters have almost become a free-range state.
It wasn't until Sauron had gained a foothold in Mordor that he noticed these tentacle-wielding sea monsters, which he sent to travel off the coast near the mainland, attacking human and elven ships. The one in front of Moria's door, probably attracted by some reason, went down the river into the hinterland of Middle-earth.
Well, about the water monitor will be introduced here for everyone, thank you for reading!
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