In the recent Doctor Who episode "Legend of the Sea Devils," the creatures returned to the show for the first time in decades. Since the show's return in 2005, successive show-runners have brought back a cavalcade of classic nasties from its original 1963-1989 run.
Though some nods to the past have been minor - the Autons, the Macra, Alpha Centauri - some of these enemies have proven themselves to truly stand the test of time.
Sea Devils (& Silurians)
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The Silurians and Sea Devils are primeval saurian species. They were the most dominant, intelligent creatures on planet Earth during the time of the dinosaurs. Indeed, the Silurians tamed a T-Rex to protect their underground base in their debut appearance (1970). Both the Silurians and Sea Devils hid - underground and beneath the sea respectively - when Earth was affected by a major environmental disaster.
As they slumbered, humans evolved to become the new dominant species. To these reptilian races, humans are invasive upstarts. The reboot series has concentrated on this loss of their territory sympathetically, with the Doctor’s attempts to broker relations between them and humans often tragically doomed.
Cybermen
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Despite being Doctor Who’s second most iconic classic monster, it’s hard to pin down precisely what the Cybermen represent. In their 1966 appearance, they were intended by creator Dr. Kit Pedler as a commentary on cybernetics - humans molded into machines. Later, they were arguably stand-ins for a Soviet-style “communal” force that operated on inhuman machine logic.
The reason they work, then, is that at heart they’re mechanical zombies. The reboot has understood this horror time and again, making them corporate upgrades (their return in "Rise of the Cybermen"), parallel evolution ("The Doctor Falls"), and a zealot’s idea of the afterlife ("Ascension of the Cybermen").
The Master
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Debuting in the 1970s when the Third Doctor was stuck on Earth, the Master was intended by show-runner Barry Letts to be the Moriarty to the Doctor’s Sherlock Holmes. Fittingly, then, the first Master was a classy, scheming mastermind who was often undone by the Doctor’s cleverness. As the series went on, however, the Master lost his classy form and became a shriveled corpse of a man who had to steal other people’s bodies just to survive.
The Doctor and the Master’s childhood friendship turned deadly rivalry has been fruitful ground for the reboot to utilize. The Master and the Doctor have been survivors of a genocidal war ("The Sound of Drums"), friends who dearly wish to reunite ("World Enough and Time"), and children gaslit by their home planet’s founding fathers ("The Timeless Children").
Davros
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Davros was first seen in 1975 creating the Daleks for the purpose of ending a war with unequivocal genocide. Since then, Davros’ relationship with his “children” has been proud but strained. Sometimes, as in "Destiny of the Daleks," they have come to him for strategic help. Sometimes, though, the Daleks are quite happy to lock him up or attempt to murder him for acting like their stern father.
Davros is a significant figure because he represents the Daleks’ hatred whilst being an actual character who can engage in discussions about morality. When he returned in 2008, he was quick to criticize the Doctor for fashioning his friends into soldiers - very much the Doctor’s “children” in conflict with Davros’ own. Yet a later story revealed that the Doctor had saved Davros’ life as a child, showing immense kindness for a man for whom kindness is a sign of inferiority.
Zygons
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Wanderers whose home was destroyed by a stellar explosion, these fetal, suckered creatures were first seen in 1975 feeding off the milk of the Loch Ness monster. Apart from utilizing this large sea creature as a weapon, the Zygons’ main talent is body-snatching - impersonating important people in a bid to invade Earth.
Yet their status as a displaced species meant that their return in the reboot was much more sympathetic. In "The Day of the Doctor," the Zygons’ impersonation of UNIT officers is tied into an overarching theme of identity and what makes someone themselves. By the Twelfth Doctor’s time, the Zygons were hidden refugees hunted by bigoted humans, and an angry splinter group decided to end the human threat once and for all before the Doctor stepped in and forced co-existence once more.
Ice Warriors
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These scaly, intimidating creatures derive from Mars. Despite its cold climate being amenable to their needs, they feel their planet is dying. In their first appearances in the 1960s, the Ice Warriors intended to capture technological devices that could transition Earth’s atmosphere into one in which they could populate. In the 1970s, however, the Ice Warriors were seen in a galactic federation operating as a responsible delegation - and thus the Ice Warriors’ central trait, their honor, was born.
Both of their reboot appearances - "Cold War" and "Empress of Mars" - operate on this principle. The Soviets and the British imperial army, respectively, come to blows with the Ice Warriors, and the Doctor appeals to their better nature to call off hostilities. They may look threatening, but the Ice Warriors are the most likely classic villains to cease killing.
Sontarans
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The Sontarans live for war - they were bred for it. Their trollish, ugly appearance betrays that they’re all clones of the same warrior template. Indeed, the Sontarans are so grotesque that they circle around to being funny. These space goblins arrived in "The Time Warrior" with a degree of seriousness, but later appearances had them very much being the butt of the joke, such as in "The Two Doctors."
The reboot’s versions of the Sontarans have followed suit. Upon their return in 2008’s "The Sontaran Stratagem," they were retooled as chanting lunatics who sounded like British generals. Later, an individual Sontaran named Strax was introduced, who has given up his warring ways and become a nurse - or so he likes to think. Much of the comedy around this character revolves around his inept attempts to wean off his love of battle. Finally, the Sontarans were seen fighting in the Crimean War in "Flux," partly to dominate Earth, and partly because their general wanted to ride a horse.
Daleks
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Doctor Who’s most iconic monsters by far, these gliding, electrified pepper pots are united by one thing: hatred. Their second story, "The Dalek Invasion of Earth," took their debut’s suggestion that they were prejudiced and turned them into full-blown space Nazis, waving their suckers like Roman salutes.
Their assumption of superiority is (sadly) just as resonant in the reboot series. For example, in "Revolution of the Daleks," they were cozy bedfellows with the UK’s government. The reboot also made them the opposing force in a war that wiped out the Doctor’s entire race, cementing them as the most significant enemies in the entire series.