Spider Parasites of Atlach-Nacha

 Atlach-Nacha sends out spider-like creatures which can take over the mind of others.

Servants of Y'golonac

Small, deformed, and eyeless, the Servants of Y'golonac dwell with him in darkness behind a great wall. Like him, they also have sharp-toothed mouths on their palms.

Voors

The Voors were a powerful and prosperous race of dwarves, predating humankind. Driven underground by the Hyperboreans and humans, they evolved into blind, albino creatures.

Travelers

Much like the Shan, Travelers invade a host's brain and take control of them. Unlike the Shan, however, Travelers are physical beings, and enter their victim's body through their mouth. They start off in a larval form (shown below) and mature once inside the host. Unlike the Shan, however, Travelers can't easily vacate their host.

Terrors from Beyond

Terrors from Beyond—also known as Astral Parasites—appear like jellyfish floating through the air... when they can be seen. Normally they occupy their own dimension, completely invisible to ours, but with special devices, they can be seen. The only problem is, as soon as we can see them, they can also see us... and they are always hungry.

Shan (Insects from Shaggai)

The Insects from Shaggai, also known as Shan, came to Earth many years ago and became stranded due to a component in our atmosphere. They are a highly advanced race which enjoy torturing other races for whatever reason. They can also phase through solid objects, which they use to enter a human's brain to tinker with it.

Rat-Things

Brown Jenkin, from Dreams in the Witch House, is a rat-thing, presumably named such because he is mostly a rat, but also has a deformed, human-like head, as well as human-like hands instead of feet. Rat-Things are primarily witches' familiars and were possibly once human. 

Rat People

Once human, Rat People have degenerated to a near animal state. I get the impression from their write-up in the old Malleus Monstrorum that they are even less human than those pictured below, as it says that they sometimes walk on all fours and has no mention of weapons or even clothing.

Miri Nigri

The Miri Nigri are mute dwarves created by Chaugnar Faugn from the flesh of toads. They tirelessly serve Chaugnar Faugn in his lair, only leaving twice a year to acquire sacrificial victims.

Martense Kin

The Martense family, through generations of inbreeding, degenerated into animalistic monsters. They left the surface world to dwell underground in labyrinthine warrens, shunning the light—though it doesn't actually hurt them.

Goatswood Gnomes

The Goatswood Gnomes are statues that have been enchanted to life. They are dormant during the day, but come to life at night to carry out vile and devious acts.

Desh

The Desh are creatures from an alternate dimension that can be summoned to earth through the brain of a sentient being. They come in many different shapes and sizes, two samples of which are pictured below, named Lesser and Greater Desh. Lesser desh appear as large silvery tadpoles with limp, toothless mouths, and with a row of dark, bead-like eyes across their narrow heads. Their long limbs are tough and springy. Greater desh have slim, fish-like bodies with four long limbs knotted with cords of stringy muscle. Wide mouths bristle with curved, six-inch teeth. The top of the sleek head is crowned with a bundle of 2 to 8 lidless, plate-shaped eyes arranged in a crescent.