(Hey look, I can make posts that aren't just His Majesty the Worm shilling. I still am playing games and thinking about them!)
For my current Dolmenwood game, my players said they wanted to all play witches. To play to this gimmick, I wrote a magical familiar subsystem. Rules below in a His Majesty the Worm parlance, but can be adapted to any OSR system without difficulty. Lots of inspiration for this system came from the unparalleled Goblinpunch.
A thing I really like about this subsystem is how it creates trouble for the players. Each familiar wants something weird and outrageous, which pushes the characters into interesting/dangerous situations to try and keep the little shit happy.
Also, because this system creates such interesting little personalities, the players never have the "Oh right, I have a pet I forgot about" moment. Actually, they're penalized with an in-character justification if they ever have that realization as the familiar has felt neglected and slipped away into the dreaming world (see below).
The one big con about this system is that I have to talk in a lot of funny voices to handle the bonded NPCs that the players all have now. Exhausting on my vocal chords.
Binding your Familiar
To bind a familiar to your service, you must go on a vision quest into the lowest realms of Dreaming (the parts that are tangent to the upper levels of Hell). Find some drugs that can get you there, if you have no spells that can do it.
Creating your familiar is like going to Build a Bear Workshop. Choose three representative pieces of the familiar and combine them together. (The art I made for my players for this is amateurish, but was accepted with applause, which made me feel good.)