In a recent episode of Star Trek: Lower Decks, Ensign Mariner is approached by a hostile being that appears to be pure energy (she tricks it into using up nearly all of its energy to produce a fancy tricorder).
In classic Star Trek: The Original Series, there are at least two episodes that feature such energy creatures. For instance, in “Errand of Mercy” such creatures pose as simple folk inhabiting the planet Organia. In addition, in “The Lights of Zetar”, the eponymous lights are actually independent creatures.
I know I have seen other such creatures throughout science fiction.
Are such creatures possible? As described, probably not! As Lawrence Krauss mention in his “Physics of Star Trek” an energy creature would just be light, and light travels at…well, the speed of light, so would dissipate quickly. However, it might be possible for a hot plasma (energetic ionized particles) to be contained by a magnetic field and also emit light, and thus perhaps give the appearance of being, for the most part, an non-corporeal “energy creature.” The problem with this is that sometimes the creatures themselves call themselves “pure energy.” So I rate this one as being partly plausible, but not entirely like it would usually appear in sci fi.
If you’d like to see more about such topics, check out The Physics of Science Fiction by Steve Bloom (me), available from your favorite bookseller such as Amazon (electronic or paperback copies) or you can get it from the publisher, McFarland.