Quasit

Quasits are chaotic evil outsiders. Standing about 1.5-2 feet in size and weighing about 8 pounds, they resemble tiny humanoids with spiky horns and bat wings. They are drawn to become the familiar of mages that match their demonic alignment.

Similar to its Devil counterpart, the imp, the quasit is a tiny demon which strikes from ambushes and uses shape shifting (via its Alternate Shape ability) and invisibility to enter combat at the prime moment to administer its poison. Though weak it still has the innate qualities of a demon making it somewhat challenging to adventurers, and quasits rarely attack alone or without the ability to escape.