Shoggoth

Shoggoths are enormous fictional species in the Cthulhu Mythos. The size of a subway train, they look like gigantic amoebas made of tar, with glowing eyes floating on the surface. Before native life arose on Earth, the Elder Things created the Shoggoths as living construction equipment; being amorphous, they could take on any shape their controllers desired, making them very versatile. However, the mindless Shoggoths eventually gained intelligence, and rebelled against their creators.

The Shoggoths are terrible things to behold. Even the Mad Arab, Abdul Alhazred, writer of the Necronomicon, was terrified of the mere thought that they might have existed on Earth.

"It was a terrible, indescribable thing vaster than any subway train - a shapeless congerie of protoplasmic bubbles, faintly self-luminous, and with myriads of temporary eyes forming and un-forming as pustules of greenish light all over the tunnel-filling front that bore down upon us, crushing the frantic penguins and slithering over the glistening floor that it and its kind had swept so evilly free of all litter..." -- "At The Mountains of Madness". H. P. Lovecraft