The Lobster Monster is a large monster which resembles a lobster.
The Lobster Monster resides in The Cave of Ali Baba, where it guards it's treasure.
Behind the scenes
The Lobster Monster first appeared in the DuckTales 2017 theme song. He is one of the characters specifically created for the series, who presumably does not have a counterpart in the regular universe. However, he does appear in he same location as the Roc in the Carl Barks' painting The Cave of Ali Baba.