From my searches online, I've managed to find out what exactly a Familiar was, but nothing more helpful than that. The traditional view of a Familiar being given to the witch by Satan doesn't cut it here, and pretty much the only other stuff I can find is about Wicca, which also doesn't help me.
So, what I want to know is, how could he have acquired or created this Familiar? Any mythology/folklore will do, and I'm prepared to bastardise things if I have to. If there's anything in any of the Lovecraftian/Cthulhu mythoses (mythi?), that would be even better, but like I said, I'm open to suggestions.
One thing I should probably mention now is that "magic" in the typical fantasy sense is pretty much completely nonexistant in my story. This character can't just wave a wand or say some magic words and have a Familiar appear.
Thanks in advance!
EDIT: I'm getting some fantastic suggestions- thanks so much! I should have enough to work with now, but I'm certainly not going to reject any more ideas.