Elder Things (sometimes confusingly called the "Old Ones") are a race of strange ancient aliens from the Cthulhu Mythos. They were first introduced in Lovecraft's At the Mountains of Madness (fiction).
One of the strangest of Mythos creatures, when explorers first found hibernating elder things, they were uncertain whether or not they were plants of animals.
There physical depiction is summarized in Mountains of Madness.
"Six feet end to end, three and five-tenths feet central diameter, tapering to one foot at each end. Like a barrel with five bulging ridges in place of staves. Lateral breakages, as of thinnish stalks, are at equator in middle of these ridges. In furrows between ridges are curious growths – combs or wings that fold up and spread out like fans. . . which gives almost seven-foot wing spread. Arrangement reminds one of certain monsters of primal myth, especially fabled Elder Things in [the] Necronomicon."
-H. P. Lovecraft
The elder things were radially symmetrical and dark gray in coloration. Their wings, which originally allowed them to fly, became useless after thousands of years of disuse. Eyes were located at the end of the arms of the organism's starfish-shaped head. Elder things can survive both in and out of water, and once had cities across the world.
The Elder Things were apparently very skilled bioengineers, and are credited with creating the Shoggoths among other organisms. They are noted for building great cities using shoggoths, and also using the creatures in Wars against the Mi-go, Yithians and spawn of Cthulhu.
Unlike most species of the Cthulhu Mythos, the Elder Things seem to have no religion whatsoever, and their society had even been suggested as being a socialist one. Much of Elder Thing life has been interpreted from the ruins of their great cities and the intricate cavings left on the building walls.