In the movie Aquaman, Aquaman is attacked by a sea monster who says something like: "no one has conversed with me since king Atlan" and the monster calms down, later the monster fights with Aquaman (after he claimed the trident) in the big battle.
Throughout the rest of the movie I didn't see any confirmation that the monster actually served king Atlan. Maybe I missed something, or it was meant to be left ambiguous. Any clarifying information is appreciated.