A shoggoth (occasionally shaggoth) is a monster in the Cthulhu Mythos. The being was mentioned in passing in sonnet XX ("Night-Gaunts") of H. P. Lovecraft's sonnet cycle Fungi from Yuggoth, written in 1929–30, and were expounded upon in his novella At the Mountains of Madness (1931).
"It was a terrible, indescribable thing vaster than any subway train—a shapeless congeries 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."
The definitive description of shoggoths comes from the above-quoted story. In it, Lovecraft writes them as 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. An average shoggoth measured fifteen feet across when a sphere, though the story mentions ones of much greater size.
Mythos media most commonly shows them, although intelligent to some degree, dealing with problems using their great size and strength. For instance, the original one mentioned in At the Mountains of Madness simply rolled over and crushed giant albino penguins that were in the way as it pursued the characters.
The character of the Mad Arab, Abdul Alhazred, found the mere idea of their existence on Earth terrifying as many characters from Lovecraft's stories do.
The shoggoths were created by the Elder Things. Being amorphous, they could take on any shape needed, making them very versatile within their aquatic environment. Though able to "understand" the Elder Things' language, 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, the Elder Things succeeded in quelling the insurrection, but thereafter watched them more carefully. By this point, exterminating them was not an option as the Elder Things 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.
Within the Mythos, 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 At The Mountains of Madness brings up the possibility of the Elder Things being the creators, having made early life as discarded experiments in bioengineering.