A shoggoth.

The Shoggoths are a race of extraterrestrial monstrosities that were created by the Great Old Ones but later rebelled against their masters after they gained intelligence. They are massive amoeba-like creatures made out of iridescent black slime, with multiple eyes "floating" on the surface. They are described as "protoplasmic", lacking any default body shape and instead being able to form limbs and organs at will. They are one of the most terrifying creatures that ever walked the Earth. So much so, that many would classify them as "true" monsters.

These creatures were created when some of the Old Ones managed to escape the wrath of God and dwell on Earth for some time. Seeing that this planet would serve as their own residence, they began to spread about and build their kingdoms. They first started by creating their own species and kingdoms. As time passed, many other alien races visited the Earth and began to colonize upon it for their own needs, something which the Old Ones took ill towards. Soon, when an alien race landed, they created an organism that would become the shoggoths

Being amorphous, they could take on any shape needed, making them very versatile within their aquatic environment. Though able to "understand" the language of the Old Ones they had no real consciousness and were controlled through hypnotic suggestion.

The shoggoths built the underwater cities of their masters. Over millions of years of existence, some shoggoths mutated and gained independent minds. Some time after this, they rebelled. Eventually, their masters succeeded in quelling the insurrection, but thereafter watched them more carefully. By this point, exterminating them was not an option as the alien lords were fully dependent on them for labor and could not replace them. It was during this time that, despite their masters' wishes, they demonstrated an ability to survive on land.

the existence of the shoggoths possibly led to the accidental creation of Ubbo-Sathla, a god-like entity supposedly responsible for the origin of all life on Earth, though unlike the one humans are familiar, this "life" was meant to suit the chaotic nature that is connected to how the Old Ones are. However, the archangels found them and destroyed them all swiftly by creating various extinction level events, which also lead to the destruction of the dinosaurs.