The Silurians are a perfect example of this, as we have come into conflict with them for mastery of the Earth, but they are a creature that comes from the depths of our own past, not the space beyond our reach.
They are a perfect foil for arrogant Humans as they are arrogant themselves, both believing that they are the better of the two races due to the level of technology they each possess or the threats of destruction it can carry out, it is a situation that has echoes that come from our own real life battles of prejudice and pride, making it as easy source for quality drama in the terrorist age.
The effect is seen in both Silurian appearances in the classic series of Doctor Who, as in both, they draw comparisons to the old enemy agents of the past or our modern terror cell.
In Doctor Who and the Silurians, they are awoken by Human experimentation and use the Human's nuclear reactor to revive more of their kind, and eventually after a coup by a more hostile faction of Silurians, resort to using a biological weapon ti wipe out the Human population which the Doctor foils before the military moves in and blows up the caves.
When they return in Warriors of the Deep they are shown more in the light of diplomats who were tired of the peaceful path and who's goals were to launch a missile from an underwater base they were holding hostage. At least on TV, Silurians were the perfect model for the kind of real fears that we faced on the from page of the daily papers no matter what the era.
In the New Adventure Blood Heat, we got see an alternate reality where the events of Doctor Who and the Silurians took a different direction, one where the third Doctor was killed as was most of the Human population by plague leaving a small UNIT group and other stragglers the only survivors on a world returned to a prehistoric environment.
While the Scales of Injustice is a missing adventure that can be seen as a direct sequel to the same third Doctor serial as well as explaining away a ton of questions from the television story Warriors of the Deep, and ultimately shows a more peaceful Silurian approach with only a few extremists in the ranks, but ultimately it ends in bloodshed and the Big Finish audio Bloodtide, in which our whole creation as a race is explained as a forbidden experiment carried out by a Silurian scientist.
On both sides of the conflict both Silurian and Human camps have those that wish for peace and those that would rather annihilate or segregate anything not like them, making them a race perfect for the darker more intricately plotted tones of the new series of Doctor Who, allowing for a good deep plot that should have real-world implications.
Something that was shown not only in films like Alien Nation and District 9, which have similar themes of an alien culture merging with ours, but also to some of the best of the past Doctor Who episodes like the Sunmakers or more recently Bad wolf, story's that pull just enough material from the real life problems and woes of mankind that it makes them more powerful and relevant, even though they are taking place in a fantasy world.
The alien creatures in question might have feelings and a society, but deep down inside we are scared and prejudiced as a culture.
But we are also afraid because the Silurians are also the monster lurking below the ground in the fairy tale, the thing waiting in the depths of some dark cave, watching you from the shadows.
It is this too that make Silurians a better than an average monster, they are in the simple description the same old monster that has dwelt beneath our feet and our bridges for centuries, only they are more us than them, which makes them even more terrifying.
Connected to our own real life home world of Earth, the Silurians are surely one of the greatest and most thought-provoking species ever to be created on Doctor Who.