Formless Horror, the Outer Limits episode "Counterweight"

Fear is a very formless thing, or rather it has all forms.  When one is afraid every shadow has a dark intent.  Each sound is "them" getting closer.  Here is the actualization of fear.  A bunch of hopeful colonists are taking part in a grand experiment.  They are in a simulation of a flight to a nearby planet to see if it is feasible.  The trouble is that the natives would rather not have tourists.  So one comes to the experiment looking like a light snaky thing and enters into each person trying to cause fear.  When that isn't enough it occupies a harmless fern and turns it into this:

Needless to say the experiment was called on account of monster.  It's an open question whether they could have made it anyway.   The monster only exploited the weaknesses only there.