Monster

Monster was a term for a frightening lifeform.

The Doctor and monsters
The Fifth Doctor claimed to fight monsters as he "zoom[ed]" through time and space. (TV: Time Crash) The Seventh Doctor said to Bernice Summerfield that he was what monsters had nightmares about. (PROSE: Love and War) The Eighth Doctor said to Destrii that he was not scared of monsters, but that monsters were scared of him. (COMIC: Uroboros) He later repeated this claim to Elliot Northover while in his eleventh incarnation. (TV: Cold Blood) The Tenth Doctor said that monsters had nightmares about him. Jeanne-Antoinette Poisson realised that one could not have the Doctor without the monsters, but that the Doctor was worth the monsters. (TV: The Girl in the Fireplace) The Twelfth Doctor once identified himself as "the man who stops the monsters". (TV: Flatline)

Species
The Tenth Doctor called the human race monsters after Harriet Jones had the Sycorax destroyed. (TV: The Christmas Invasion) called the human race the "greatest monsters of them all." (TV: Last of the Time Lords)

Clara Oswald called the Time Lords monsters. (TV: Hell Bent)

Among the life forms which were called monsters were the mutated Yend, (PROSE: The Sons of the Crab) dinosaurs, (TV: The Underwater Menace, Invasion of the Dinosaurs) Silurians, (TV: Doctor Who and the Silurians) Aggedor, (TV: The Curse of Peladon, The Monster of Peladon) Kronos, (TV: The Time Monster) Linx, (TV: Invasion of the Dinosaurs) the Exxilon City, (TV: Death to the Daleks) the Mutos and the Daleks, (TV: Genesis of the Daleks) Erato, (TV: The Creature from the Pit) the Process, (PROSE: Cat's Cradle: Time's Crucible) the Krafayis, (TV: Vincent and the Doctor) the Gangers, (TV: The Almost People) the Minotaur, (TV: The God Complex) the Crooked Man, (TV: Hide) the Foretold, (TV: Mummy on the Orient Express) the Boneless, (TV: Flatline) the Sandmen, (TV: Sleep No More) a creature living in the River Thames (TV: Thin Ice) and the Eaters of Light. (TV: The Eaters of Light)

was aware that the Ogrons feared Rock Creatures who he described as an "orange scrotum monster", as recorded in the Masterplan Journal. (PROSE: The Secret Diary of the Master)

In 1879, Queen Victoria asked Sir Robert MacLeish to tell her about monsters, and so he told her of the Lupine Wavelength Haemovariform. (TV: Tooth and Claw)

In 1930, Lazlo believed that he had become a monster after being transformed into a pig slave by the Cult of Skaro. (TV: Daleks in Manhattan) A Hooverville sentry described the pig slaves as monsters. (TV: Evolution of the Daleks)

In 1953, Tommy Connolly described the faceless human victims of the Wire as monsters. His mother, Rita Connolly, believed her mother had been replaced by a monster. (TV: The Idiot's Lantern)

Sarah Jane Smith recalled meeting anti-matter monsters. (TV: School Reunion)

In 1987, during Mickey Smith's childhood, he described the Reapers as monsters that had come to eat him. Suzie asked him if the monsters were aliens. (TV: Father's Day)

Bog monster was a term used by Mickey Smith to refer to the Slitheen. (TV: World War Three, The Christmas Invasion)

Persons and groups
The term was also given to immoral persons or groups. Sharaz Jek called Morgus an inhuman monster. (TV: The Caves of Androzani) The Seventh Doctor thought of Heinrich Himmler's darkened office as "a cave for a monster." (PROSE: Timewyrm: Exodus)

Ianto Jones called Jack Harkness "the biggest monster of all." (TV: Cyberwoman)

Martha Jones described Richard Lazarus as a monster after he had been transformed by the hypersonic soundwave manipulator. (TV: The Lazarus Experiment)

Mr Copper observed to the Tenth Doctor that anyone who decided who was to live or die would be a monster. (TV: Voyage of the Damned)

Carla Langer called her ex-husband Paul a monster. (TV: The Mark of the Berserker)

Donna Noble called the Meep a monster after the Meep murdered Zogroth and Zreeg. (TV: The Star Beast)

Yarvell called Davros a monster when his only interest in their father's death was using the corpse for his experiments. (AUDIO: Innocence)

In culture
George Thompson was terrified of monsters and pleaded for someone to save him from them. (TV: Night Terrors, PROSE: Night Terrors, My Special Book) Gilgamesh jokingly called Enkidu a "hairy monster." (PROSE: Timewyrm: Genesys) Ada Gillyflower called the Eleventh Doctor her "dear monster". (TV: The Crimson Horror) Erik set up recordings of monsters around his cottage so that his daughter Hanne wouldn't leave it. (TV: It Takes You Away)

Sea monster was an umbrella term for mythological monsters which resided in the seas. (TV: Partners in Crime)

Vorg called his miniscope "the monster show" and "a carnival of monsters". (TV: Carnival of Monsters)

The Loch Ness Monster was a mysterious animal claimed to inhabit Loch Ness, a lake in Scotland. (TV: Terror of the Zygons)

Behind the scenes
In the DVD commentary to Fear Her, writer Matthew Graham noted that, according to then-executive producer Russell T Davies, the scribble creature was never to be called a scribble monster, as the Doctor would never call anything a monster.