Swamp monsters have been very popular in comic books since the first decade of the Golden Age. In 1940, Theodore Sturgeon's short story "It!" was published in Unknown magazine. It was about a plant monster that had grown from a human skeleton, and was the inspiration for a generation of swamp men. But these plant and mud monsters have primarily been popular in comic books; humanoid reptiles and fish-like gill-men such as the Creature from the Black Lagoon have fared better at the movies.
The folklore of North America has included swamp men...but usually these creatures have resembled sasquatch, zombies, wild men, ghosts, and even werewolves. These boogey-men tales could have been concocted by locals to keep outsiders away, or prevent children from wandering into dangerous areas.
One alleged encounter in Fouke, Arkansas, in 1971 set off a rash of sightings. The "Fouke Monster" inspired the 1972 film The Legend of Boggy Creek, and probably both Marvel's Man-Thing and DC's Swamp Thing.
Briefer may or may not have seen Sturgeon's short story, but he was probably familiar with Hillman Periodical's swamp creature, the Heap, who had first appeared in 1942 in Air Fighters #3. The Heap would later star as a backup feature in Airboy from 1946 until 1953.
Here is "Swamp Spirit" from Frankenstein #16 (Nov-Dec 1948):