An Animal who is made of magic or was exposed to magic constantly for most of its life, and has gained higher intelligence and magical capabilities. These animals choose someone who is also made of magic to become companions with. All magical races can have one, but they're most common in Witches. Fae, Vampires, and Sirens get chosen occasionally and Shifters are rarely chosen because they only use magic to shift.

Intelligent

Noticable Intelligence, Sometimes they glow when helping their companion with something magic related or when threatened. The color and behavior of the glow depends on the magic they were born with or absorbed

Depends on the race of their companion. A familiar is always very similar in personality to them. They only pick a companion they respect and get along with. So for example, Siren and Vampire Familiars would likely be most aggressive, as both species are territorial by nature.

Magic and whatever physical features they have on hand (a cat having teeth and claws etc)

Spiritual/Magical Senses

Physical Senses

They prefer sticking close to their chosen person, their individual needs depend on specific species

The non magical versions of animals that absorbed magic

I imagine they may become stronger magically. They aren't infallible and have many weaknesses. Especially those that absorbed magic instead of being born with it. They aren't a mythical creature and are the same as the creatures on earth (cats, dogs, ravens, crows etc absorbing magic). I think evolution will find a way to correct this eventually

Moderate. Magic Races get into a lot of danger and many familiars aren't born mythical.

Peep Jewel's character page for more info on my favorite familiar.