What is the name of the lake monster in the episode 'Quagmire'?

The lake monster in The X-Files episode "Quagmire" is called Big Blue.

The third-season episode follows Mulder and Scully as they investigate deaths around a lake in Millikan, Georgia. Local stories blame the attacks on Big Blue, a plesiosaur-like creature believed to inhabit the water. Mulder takes the legend seriously and suspects that the monster has moved closer to shore because the lake's declining frog population has reduced its usual food supply.

The episode deliberately plays with the difference between belief and proof. Mulder eventually discovers that an alligator is responsible for the attacks, including the apparent death of Scully's dog, Queequeg. However, the final scene shows a large creature moving through the lake, preserving the possibility that Big Blue really exists. Thus, Big Blue is the canonical name of the rumored monster even though it is not the killer revealed in the main investigation.

"Quagmire" aired on May 3, 1996, as the twenty-second episode of season three. Its extended conversation between Mulder and Scully, stranded on a rock after their boat sinks, is one of the episode's best-known scenes.