Familiars are entities that take the form of animals and help people along their journey in witchcraft.
Familiars aren't a "one time only" thing. You can have multiple familiars at once. However, familiars don't always stick around for a lifetime. If a familiar takes the form of a household animal, chances are it's there to stick around (But that's not always the case). If it takes the form of a wild animal, it may stick around forever, but it's most likely that it'll help you for a shorter period of time.
Familiars are not pets! They are conscious entities and should be respected. If your familiar is a fox who doesn't want to be trapped in your home, don't trap it in your home. They aren't all supposed to live in your house.
You don't get to pick what animal your familiar is or what personality your familiar has, you'll get what your given, a.k.a what comes to you. Your familiar could be a rat, a deer, a toad, a dog, a horse, a raven, etc. It could be anything, but you don't pick it.
Familiars usually come to you when they want to, but it is possible to contact/reach out to a familiar on your own. It'll be much easier if they reach out to you, but like I said, you can reach out to them on your own.
Keep in mind that some pets are magically inclined, but that doesn't make them a familiar! Being magically inclined and being a familiar are two separate things. Also, if an animal comes up to you because you keep feeding it, that doesn't mean its your familiar, it just wants food.
(I kinda wrote this chapter lazily, so I'll probably change some stuff later)