Wikipedia
In European folklore and folk-belief of the Medieval and Early Modern periods, familiar spirits (sometimes referred to simply as “familiars”) were supernatural entities believed to assist witches and cunning folk in their practice of magic. According to the records of the time, they would appear in numerous guises, often as an animal, but also at times as a human or humanoid figure, and were described as “clearly defined, three-dimensional… forms, vivid with colour and animated with movement and sound” by those alleging to have come into contact with them.
Familiars are neither human nor are they animals
They can accept a partnership with a magician/witch. Through which they get part of the magic of the mage. Being helpers and guardians in return
They have three/four forms:
a) the normal animal form in which only the oldest can talk.
b) their human form in which they keep scars/hair color/eye color from their animal form and
c) their shadow beast form, for most only accessible when their mage helps. Giant shadow beasts still in the form of their normal animal form.
d) a mix of human and shadow beast form, like human but with wings and claws. Not all have this form
Only young familiars can be tamed against will, what is still a bad idea since the stronger the bond between mage and familiar the stronger both are.
They can get much much older than humans and even Mages they can get centuries old with always changing masters as long as they have some source of magic. Either a bond partner or just some magical place
Some Mages and Familiars fall in love, what isn't always the case and is frowned upon by many
Of course they have the senses of their animal form in all forms, especially strong in their beast form, though their weaknesses too. A bat might have poor eyesight for example
