Alligator (1980)

Ramon the alligator is flushed down the toilet as a baby and grows into a gargantuan monster by eating the corpses of laboratory animals who have undergone dubious hormone experiments, thus providing all the ecological and social subtext that one could possibly wish for, even if one doesn't normally go for films about giant alligators eating people left, right, and center--which is the inevitable and tragic result of Ramon's decision that the outside world looks rather more interesting than the sewers....