b.e.m., n. — Green’s Dictionary of Slang

b.e.m. n.

[abbr.]

(orig. US) bug-eyed monster/monsters; a popular category of SF writing and described as such by fans.

1951		F. Brown Space on my Hands 85: ‘B-bems?’ asked Dorothy. ‘You mean you are b-bug-eyed monsters? That’s what Elmo means by Bems.’.
1955		R.M. Lindner Fifty-Minute Hour (1986) 280: My passage from BEMs through Burroughs to Wells.
1982		Time 31 May 60: At first startled glance [...] you could call him [i.e. the movie extraterrestrial E.T.] a BEM [HDAS].
2002		Small Blue Planet Toys 🌐 Rev (as in Reverend) Bem (as in Bug Eyed Monster), a Magog, has a real name, but it’s an unpronounceable screech in his native language. The closest English translation is Red Plague.