Villain(s)  Mutated alligator

Actor(s):  John Agar, Ralph Baker Jr., Bill Thurman, Dorothy Davis

Brief Description  The movie you are referring to is actually titled Night Fright (1967), though it was distributed with the alternate title Fright Night in some markets (and later repackaged in the UK on VHS as E.T.N.: The Extraterrestrial Nastie). Directed by James A. Sullivan (who famously served as the editor for the notorious cult film Manos: The Hands of Fate), this low-budget, independent sci-fi horror film was shot in Dallas, Texas. It heavily blends the tropes of 1950s "creature features" with late-1960s youth culture.

Plot:
The story kicks off with a fiery object crashing into the woods near a rural college community in Texas—an area the local youth call "Satan's Hollow". A young couple making out in a parked car hear a radio report about a suspected UFO crash shortly before they are brutally attacked and killed by an unseen entity. The next morning, Sheriff Clint Crawford (played by B-movie veteran John Agar) arrives at the scene, only to find the area heavily cordoned off by mysterious government agents. It is revealed that the crash wasn't an alien spaceship, but rather a NASA rocket from a secret space experiment called "Operation Noah's Ark".

The rocket, carrying several test animals around the moon, was bombarded by intense space radiation, mutating an ordinary alligator into a giant, unstoppable, bullet-proof monster. As the mutant creature breaks out of the containment zone, it begins terrorizing the community. A large group of local college students ignore the danger and head out into the woods to party near the crash site. The monster stalks and kills several students, deputies, and locals. Realizing conventional weapons are useless against the beast, Sheriff Crawford enlists the help of a scientist, Professor Alan Clayton. Together, they devise a plan to lure the creature using a mannequin decoy and destroy it with a dynamite trap.