Old Ones
Old Ones is a term used in fantasy games for the culture that was first on the scene in creation, before the limits of recorded history in the current setting of the game. This culture is always described as awesomely powerful, responsible for shaping entire landscapes, sinking continents beneath the waves, or giving birth to new races. An important feature of a race of Old Ones is they aren't around anymore -- or if they are, it's in some degenerate form (reduced to stone-age beasts, or a secluded colony hiding from the world, or one sleeping god-like individual).
Precursors is the sci-fi version, but doesn't carry the connotation of malevolence.
Call of Cthulhu
There were many "Old Ones" in the Cthulhu Mythos, and thus the Call of Cthulhu RPG. It's most commonly used to refer to one of the following two:
The Great Old Ones were alien beings of colossal size, and not exactly normal physics. All the Great Old Ones have been slain or imprisoned by Elder Gods, some entombed in planets like Earth. Some theorize that the Great Old Ones are imprisoned for the crime of practicing blasphemous black magic, and the Great Old Ones are just waiting to break out and avenge themselves. Others theorize the Great Old Ones submitted willingly to imprisonment, as a way to sleep away the aeons "until the stars are right" and they can resume their awful practices.
The race of Old Ones (or Elder Things) were extraterrestrials that traveled between the stars, and set up a colony on primordial Earth. They built a city on the southern polar continent and experimented with the local biology to create a servant caste. Their discards were tossed to the other continents (the Cambrian Explosion? The Cambrian Explosion pre-dated life on land.) and settled on their ideal lowest-class citizens: the Shoggoths, amorphous blobs of eyes and toothy mouths. The Shoggoths formed a rebellion and overthrew their masters, imprisoning them in suspended animation tombs of ice sealed with Elder Signs.
Forgotten Realms
Long long time ago, a lizardman type race had a huge empire that enslaved all the "warmbloods." It was probably the Sarrukh, mentioned in the Serpent Kingdoms splatbook. They're supposed to be extinct now, but it would make a decent BBEG. The Story of the First NeverWinter Nights explored this but made it all very confusing and disappointing.
Traveller
The old-ones in Traveller are called the "Ancients," and their sites can be found in any random place. The architecture can vary wildly, from habitrail tunnels hermetically sealed and protected from a benign environment, to hollowed-out crystals hundreds of feet high with no ladders or stairs. None of the sites have any documentation, many of them expect visitors to be able to fly or use telepathy, and all of them are empty and inert... except maybe whatever one your adventurers carelessly step into . Each Ancient site predates the literacy of every civilization, and doesn't seem to have anything in common with other Ancient sites other than being built for inhabitants that are 1-2 metres tall. Ancient sites make for good dungeon romps in what is otherwise a hard sci-fi campaign about making mortgage payments on your spaceship.
The Ancients also explain why there's so many humans in space. The first great human empire was the Vilani empire, built by the humans that first discovered faster-than-light travel. They were pretty surprised to find out there were other aliens around; even more surprised when they encountered another FTL space-faring empire of humans called the 'Zhodani.' Scientists concluded sure that the two-arms & two-legs body shape was just naturally better for most environments, and stopped being surprised that everyone was "humanoid" shaped. This all went to shit when a third race of humans showed up with FTL drive, calling themselves the 'men of Sol' ("Solomani" in Vilani), and it turned out that they came from a world where the entire ecosystem was integrated, not evolving in parallel like everywhere else. Vilani scientists were certain this was a sign that the Solomani were an engineered race of humans made by the Ancients, what with everything fitting together like clockwork. Instead, they discovered the opposite: the undeniable conclusion that Zhodani humans, Vilani humans, and all the other humans from minor races were "seeded" from Sol-3 and transplanted to every other world by the Ancients. Even the biology of the lupine Vargyr major race were seeded from this 'Earth' world. This gives the Solomani a superiority complex that makes them pretty insufferable to be around.
Warhammer Fantasy
Psychic frog men from beyond the stars; they put a warp-gate at each of the poles and shifted the planet's orbit to something nicer for their amphibian biology. All that's left of them in present day are the degenerate & decadent rulers of the Lizardmen armies, the Slann.
In older editions of 40K and WFB, these were the same Old Ones as the Old Ones below. In fact, the newest edition of the WFB rulebook specifically mentions their big silver space-ships, suggesting that that's still the case.
Warhammer 40,000
The Old Ones were an extremely advanced and powerful tech-sorcerer race of beings (probably lizardmen) back when the grass was green, the Warp was nice, and the Emperor wasn't born yet. They made other races like in that vidya gaem Spore.
Then they met the Necrontyr, and all hell broke loose.
The Necrontyr, being miserable piles of shit with miserable short lives envied the Old Ones and started to war against them.
The Old Ones fucked them seven ways 'till Sunday until the Necrontyr, who realised they were not strong enough to win, discovered the Star-Gods, the C'tan, sort of ghostly star vampires made of ethereal matter. They had no idea what the warp was.
Blah blah sacrifice our souls and live as cold metal automatons blah blah fight the Old ones.
The C'tans said, "No Necrontyr, YOU ARE THE NECRONS." And then Necron was a grimdark.
At this point the Old Ones supposedly created other races to ROW ROW fight the Necrons: The Eldar, the Orks and the Jokaero which are like Space gorillas like that movie Planet of The Apes, but thought humans should just be evolved a little, with psykers being rare. That's right, mutant psykers suffer because they are far too marginalised in the gene pool, shunned by society due to awkwardness and so the Imperium suffers. Well done Old Ones, well done.
Why the frick did you make the goddamn sphess elves out of us if you were gonna cut off our metaphorical dick afterwards?!! For beings that are smart enough to create brand new sentient races, they have absolutely no common sense at all.
All the galaxywide genocides made the then-silent Warp into a copy of Catholic Hell, all full of FIGHTAN, KAY-OSS, DEMONS and FAIL.
In the end Enslavers, complete Krell and Illithid rip-offs came out of the Warp and buttfucked every last Old One (and good riddance to them, the cocksucking god-wannabes). The when the last Old One was gone, Necrons rebelled to C'tan, broke them to shards, enslave this shards in pokeballs, tesseract labyrinths and then went to sleep.
The Old Ones themselves, seeming to have done one Acid strip too many, promptly disappeared from the scene, except playing the video game Spore. Fuck off you shitcunts, all the grimdark and fail is your fault. And if one of you somehow managed to survive, we'll throw you into the Eye Of Terror ourselves.