Doctor Who features many iconic and memorable monsters and villains, a lot of which The Doctor has come across multiple times. Some of these monsters like the Daleks and Cybermen have actually been tormenting The Doctor since the Classic era of the show - and are sure to continue to appear as long as the Time Lord is on screen.
However, some of The Doctor's adversaries are more low-key and have only appeared in the show once, monster-of-the-week style. While these villains may not be as iconic as some others, many of them are just as terrifying and pose huge threats in their respective stories.
Reapers
In season one, The Ninth Doctor and Rose go back in time to visit her dad on the day he died. Despite The Doctor's strict instructions, Rose takes matters into her own hands and decides to save her father's life.
Rose's decision brings with it The Reapers, who are huge winged creatures who consume people in an attempt to fix the wound in time. The creatures wreak havoc and even consume The Doctor at one point. Eventually, Rose's father sacrifices himself to defeat The Reapers and while The Doctor has never encountered these creatures again on-screen, considering the amount of paradoxes and time meddling in the show, they could definitely appear again.
Werewolf
In season 2, The Tenth Doctor and Rose encounter a werewolf (or an alien that resembles one, anyway). The episode itself is a fun romp and while the CGI is a little outdated, the werewolf is a solid threat.
The scenes where Rose is trapped in the basement are genuinely tense, and The Doctor’s admiration of the creature is delightfully true to character. The suggestion that Queen Victoria might have been scratched by the werewolf leaves the door open for a possible follow up too. Taking a creature from folklore like the werewolf and giving it a Doctor Who spin makes this one of the more exciting one-time villains.
The Krillitane
The Doctor and Rose also encounter The Krillitane in season 2. During the episode, Sarah Jane Smith, who had one of the saddest companion departures in Doctor Who, is also reintroduced to the series.
Beloved British actor Anthony Head portrays the leader of the creatures, and his scenes with The Doctor beside the swimming pool are a real tour de force. While the creatures have been revisited in expanded media, they are yet to reappear in the main show. As The Krillitane take different parts from different species, they could have a completely new design if they were to ever be reintroduced and explored further.
The Dream Lord
The sinister Dream Lord appears in the TARDIS with a huge dilemma for The Eleventh Doctor, Amy, and Rory. The trio find themselves trapped within two different realities: one where they are in grave danger on board the TARDIS and another where they are being targeted by sinister old people in the real world.
Throughout the episode, The Dream Lord terrorizes the trio but particularly targets Amy. Eventually, after Rory dies (for the first time of many) Amy decides to take her chance and sacrifice herself. The twist that The Dream Lord was part of The Doctor all along is a great end to the episode and means this villain could be used again to show the darker side of the main protagonist.
The Futurekind
The Doctor, Martha, and Jack find themselves at the end of the universe in the season 3 episode, Utopia. When they first arrive, the landscape seems desolate and empty; however, it soon becomes apparent that they are not alone. The trio are pursued by the humanoid and carnivorous Futurekind across the wasteland.
The episode is mostly used to set up the return of Classic villain, The Master and while Utopia is one of the best Master stories from the show, it leaves little time to explore The Futurekind. The creatures are understandably sidelined in the episode but it would be interesting to learn more about them and their origins in the future.
House
In one of the most emotional episodes from the show, an entity known as House tries to feed on the TARDIS. The episode is particularly memorable for being the only time The Time Machine has taken on a humanoid form and interacted with The Doctor.
Rory and Amy are left trapped inside the TARDIS as House uses its dimensions and unique properties to torment them. House has killed many Time Lords previously and callously taunts The Doctor about this which immediately solidifies it as a powerful threat. Its attempt to separate Amy and Rory and turn them against each other are what makes this low-key villain so terrifying.
The Midnight Entity
Unlike many other low-key Doctor Who villains, The Midnight Entity is one that works best as a one-off threat and probably shouldn't be revisited. Despite only appearing in the show once and its true form never being seen, the entity is still one of the best Doctor Who monsters.
The episode is haunting and psychological and places The Doctor in more danger than ever before. The humans in the episode all turn against each other and almost kill The Doctor which is what makes the entity so menacing. The voice stealing entity from Midnight is one of the creepiest villains from the show's history.
The Minotaur
The werewolf isn't the only mythological creature Doctor Who has tackled; another memorable instance of this is in the season six episode, The God Complex. The alien Minotaur feeds on people's worst fears which include Weeping Angels, embarrassment, failure, and being left behind.
The Minotaur itself is one of the most striking monster designs from the show and, just as he was when he came face to face with the werewolf The Doctor is in awe of this creature. The Shining-esque setting sets the stage for a truly creepy episode and fans are given an insight into just how much The Doctor fears for his companions' safety. Ultimately, The Minotaur is a brilliant threat that truly tests the main characters of the show.
The Foretold
At an extremely tense point in their relationship, The Twelfth doctor and Clara find themselves aboard the outer space counterpart of The Orient express. The episode starts out like a Christie-style murder mystery with passengers on the space train being picked off one by one.
It soon becomes apparent that the passengers are being stalked by a Mummy who kills them after 66 seconds. Watching the passengers beg for their lives as the creature approaches is harrowing. This episode provides a clear look into the psyche of The Twelfth Doctor and knowing that each victim only has 66 seconds to live once the Mummy appears make this adversary particularly threatening.
The Fisher King
In one of the best two-part stories from Doctor Who, the crew of an underwater mining facility are being killed and seemingly turned into ghosts. Timey-wimey stories became synonymous with Moffat's era of the show and this story is no different, exploring the idea of the bootstrap paradox.
While the hollow-eyed ghosts are chilling, particularly when The Doctor himself becomes one, the mastermind behind the whole thing is The Fisher King. This creature towers over The Doctor in stature and its appearance is grotesque and truly monstrous. Just enough of the Fisher King is shown for it to be effective without becoming overexposed.