Nightmare at 20,000 Feet - S5-E3
Trivia: Rod Serling set up a practical joke on writer Richard Matheson when the two were flying to San Francisco aboard a propeller-driven plane. Serling collaborated with the airline to tape a poster blow-up of the "Nightmare" gremlin's ugly face to the outside of the plane window. Just as Serling prompted Matheson to open the curtain, however, the plane's engines and props fired up, blowing away both the poster and Serling's intended gag.
Nightmare at 20,000 Feet - S5-E3
Trivia: Writer Richard Matheson says he was pleased with most of Twilight Zone's film version of his short story - except for the gremlin. He'd conceived it as a dark, creepy and nearly-invisible humanoid figure. "But this thing," he complains, "looked more like a panda bear."
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.