Curse of the Swamp Creature (1966) - Larry Buchanan

Deep in the rural Texan swamps, reclusive and ruthless wife-abusing mad scientist Dr. Simond Trent is conducting experiments in his laboratory on the local impoverished voodoo-worshiping black “natives” in an attempt to discover the secret to reversing evolution, feeding the failures to the alligators he keeps in his covered outdoor swimming pool.

When a party of oil surveyors comes upon his isolated yet strangely suburban-style home he decides to take the final step and turn the duplicitous female leader of the expedition into a grotesque and virtually indestructible amphibious “fish man” so that he can take his revenge upon the world…