Monster was a term for a frightening lifeform. Mickey Smith referred to bog monsters.

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)

Among the life forms which were called monsters were 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) Reapers, (TV: Father's Day) the Pig slaves, (TV: Evolution of the Daleks) 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)

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

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) Carla Langer called her ex-husband Paul a monster. (TV: The Mark of the Berserker) The Tenth Doctor called the human race monsters after Harriet Jones had the Sycorax destroyed. (TV: The Christmas Invasion) The Master 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)

George was terrified of monsters and pleaded for someone to save him from them. (TV: Night Terrors) 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)

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