“Carter could now distinguish moving figures on the noisome wharves ahead, and the better he saw them the worse he began to fear and detest them. For they were not men at all, or even approximately men, but great greyish-white slippery things which could expand and contract at will, and whose principal shape - though it often changed - was that of a sort of toad without any eyes, but with a curious vibrating mass of short pink tentacles on the end of its blunt, vague snout. These objects were waddling busily about the wharves, moving bales and crates and boxes with preternatural strength, and now and then hopping on or off some anchored galley with long oars in their forepaws. And now and then one would appear driving a herd of clumping slaves, which indeed were approximate human beings with wide mouths like those merchants who traded in Dylath-Leen; only these herds, being without turbans or shoes or clothing, did not seem so very human after all. Some of the slaves - the fatter ones, whom a sort of overseer would pinch experimentally - were unloaded from ships and nailed in crates which workers pushed into the low warehouses or loaded on great lumbering vans.” – 
Inhabitants of the Dreamlands, these beings trade in their black galleys between the Dark Side of the Moon and the disreputable port of Dyath-Leen and other ports in the Dreamlands.
The Moon Beasts have a permanent settlement on the Nameless Rock in Cerenerian Sea, which seafarers avoid. Whether the Moon Beasts have other settlements in Dreamlands is unknown. They may have been related to the late inhabitants of the now destroyed city of Ib. Their origin and description matches that of the moon beasts:
It is written on the brick cylinders of Kadatheron that the beings of lb were in hue as green as the lake and the mists that rise above it; that they had bulging eyes, pouting, flabby lips, and curious ears, and were without voice. It is also written that they descended one night from the moon in a mist; they and the vast still lake and gray stone city lb. However this may be, it is certain that they worshipped a sea-green stone idol chiseled in the likeness of Bokrug, the great water-lizard; before which they danced horribly when the moon was gibbous. And it is written in the papyrus of Ilarnek, that they one day discovered fire, and thereafter kindled flames on many ceremonial occasions. But not much is written of these beings, because they lived in very ancient times, and man is young, and knows but little of the very ancient living things. 
Moon beasts trade rubies of unknown origin for human slaves and gold. They use the Men of Leng (who may be tcho-tcho) as their go-betweens in Dyath-Leen or any human interaction.
The Cats of Ulthar are the moon beasts’ arch-enemy and can defeat a single moon beast in massed attacks.