Familiars are supernatural creatures that appear in the guise of animals to help spell casters with their work. They appear in AD&D and later versions of the game (d20 et al) but aren't included in B/X.
I think familiars are a popular feature of the game for many players and I'd like to make them available for my favourite version. I've decided the best format for familiars isn't a spell, but rather a monster entry. I'll leave it to a later date to discuss the details of acquiring a familiar.
In the Familiars PDF there are 3 types listed: Spirit, Animal and Daemon. Spirit familiars only manifest when the spell caster concentrates on them, while Animal and Daemon familiars are corporeal and ever present companions. Daemons are more powerful than regular animal familiars but they don't come without their price...
Unlike real animals a familiar is not destroyed when reduced to 0 hit points. Instead they are banished and will reappear the next evening (Spirit Familiar) or during the Full or New moon (Animal and Daemon Familiars). However due to their tie to the spell caster, the banishment of a corporeal familiar can be a dangerous occurrence.