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.

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.