Medieval-aged Muggles used to think that familiars were demons that witches summoned with dead corpses, thus the corpses came to life with the demon inside it and would help the witch. This, of course is not true, a familiar is similar to a pet but is more clever and has a loyalty and liking to a particular witch or wizard such as Albus Percival Wulfric Brian Dumbledore and his phoenix Fawkes.

Familiars have co-existed with wizards for thousands of years but there is some evidence that Archimedes, Merlin's owl was the first familiar.

An Old Muggle Painting Of Familiars