Yuggs

Yuggs are large, aquatic, pale grey worm-like creatures up to four feet in diameter and twelve or more feet in length. Yuggs have a large, round sucker-mouth similar to a lamprey or hagfish. This mouth contains several rows of hornlike teeth and is surrounded by six small tentacles six inches to two feet in length. Like snails, they leave stinking trails of slime behind them.

Shugoran

The Shugoran race look like smaller versions of Nyarlathotep's Shugoran form. They are amphibious and have black, catfish-like hides, with small wings—or fins—and long proboscises.

Shoggoths

Shoggoths are possibly Lovecraft's most popular non-deity monsters. They have amorphous forms and are nigh on unkillable.

Petesouchi

Petesouchi start life as regular humans, but after adolescence, begin to change into a large crocodile, though retaining their human mind. Like Deep Ones, not all Petesouchi make the change, while others may only partially do so.

Spawn of Othuyeg

The Spawn of Othuyeg are simply smaller versions of their sire, meaning they are a mass of white tentacles surrounding a single eye... so basically a white version of Cyaegha (hence the usurped image).

Nioth-Korghai

Nioth-Korghai appear octopus-like, but are in fact a dark-orange, "bundle of fungoid creepers joined together at one end"—so essentially tentacles without a body.

Hell Leech

While not identical, it seems that Hell Leeches are a reworking of Watchers from the old Malleus Monstrorum. They are both giant leech-like creatures that dwell in murky lakes and rivers. Unlike the image below (it was the closest representation I could find), they both also have tentacles extending from near their mouths and both have paralysing poison.

Spawn of Hastur

Octopoidal, aquatic creatures with unspeakably hideous faces that dwell in the foul and murky lake of Hali with their sire, Hastur. Like him, they may be summoned to Earth, but only when Aldebaran is in the sky.

Spawn of the Green Abyss

One of the many amorphous creatures of the Mythos, the Spawn of the Green Abyss are aquatic by nature, intelligent, and have the ability to hypnotise victims with a song.

Dwellers in the Depths

Dwellers in the Depths are aquatic creatures, and like the Deep Ones, have cities beneath the Earth's seas and rivers. They serve the aquatic Great Old Ones such as Cthulhu, Dagon, and Zoth-Ommog.

Deep One Hybrids

When a Deep One and human mate—as is the Deep Ones' way—the child is born human. It is not until puberty that they begin the change into a Deep One. This transformation takes years, or even decades, so during this time they are referred to as "hybrids".

Deep Ones

Deep Ones are humanoid fish-frogs that live in cities under the sea—though they can also survive just fine on land. They also live forever unless killed.

Dark Sargassum

A Dark Sargassum is a Dark Young that has taken to living in water. They appear at first glance to be a floating mass of black, rotting seaweed.

Thralls of Cthulhu

Once human worshipers of Cthulhu, they chose to partake in rituals that transform them into bloated, grey shadows of their former selves, with vestigial tentacles around their mouth (or head). In return, they are given immortality.

Star-spawn of Cthulhu

Cthulhu's spawn are basically just smaller versions of himself. Some were trapped in R'lyeh when it sank, though many were not.