A bug-eyed monster was a generic term for an alien creature.
Both John and the First Doctor called the Kleptons bug-eyed monsters. (COMIC: The Klepton Parasites [+]Neville Main, TVC comic stories (1964-1965).)
Sarah Jane Smith, during an argument with the Fourth Doctor, said that she was sick of, among other things, being savaged by bug-eyed monsters. (TV: The Hand of Fear [+]Bob Baker and Dave Martin, Doctor Who season 14 (BBC1, 1976).)
Captain Cook acquired a set of two-headed coins from a bug-eyed monster on Leophantos. (TV: The Greatest Show in the Galaxy [+]Stephen Wyatt, Doctor Who season 25 (BBC1, 1988-1989).)
Ace told Prof. Rachel Jensen that anyone who spent time with the Doctor eventually got used to bug-eyed monsters. (AUDIO: 1963: The Assassination Games [+]Error: Code 2 - no data stored in variables, cache or SMW.) She once asked the Seventh Doctor why bug-eyed monsters always chose to invade Earth. (PROSE: The Left-Handed Hummingbird [+]Kate Orman, Virgin New Adventures (Virgin Books, 1993).)
Sam Jones called a Janusian a bug-eyed-monster upon seeing it on the TARDIS scanner. (PROSE: The Janus Conjunction [+]Trevor Baxendale, BBC Eighth Doctor Adventures (BBC Books, 1998).)
The Eighth Doctor, upon first seeing the Face-Eater, noted that UNIT would have called it a bug-eyed monster. (PROSE: The Face-Eater [+]Simon Messingham, BBC Eighth Doctor Adventures (BBC Books, 1999).)
Clyde Langer, while distracting Lieutenant Koenig and his Nazis, claimed that he'd fought hideous bug-eyed monsters and defeated them all with his mobile phone. (TV: Lost in Time [+]Rupert Laight, The Sarah Jane Adventures series 4 (CBBC, 2010).)
Behind the scenes[]
In Sydney Newman's concept for Doctor Who, bug-eyed monsters were to be avoided. Because of this, he initially opposed the inclusion of the Daleks; however, their popularity ended up ensuring that the series would be successful.