-_- The "Familiar Spirit" is not a spirit in animal form, it's a spirit. Thats why it's called the "Familiar Spirit." The Familiar Spirit will come to you once you are ready for it. Two very different concepts surround the idea of the Familiar Spirit, the first being that the Familiar is a part of you already. When you find yourself one, talking to yourself and answering yourself (because keys be honest here, EVERYONE does it), it's said the voice of the Familiar Spirit is the one which responds. This relationship is seen as a very close, perhaps even sexual one. It's based in self love and growth.
The second concept states the Familiar Spirit is an outside entity which will revels itself to the witch in question when the witch is ready. It is this spirit the witch works most commonly with, constantly communing with it, giving offering, and working all forms of magic with its aid. The Familiar will aid the witch by communing with them in the Otherworld, giving boosts to their spells, and defending the witch from any who would do them harm. A witch with a strong relationship with their Familiar Spirit need fear no other witch or spirit. The Familiar Spirit in essence becomes a Guardian, Partner, and Counsellor.
Both concepts have their merits, it really comes down to what you believe as a person. I ascribe to the latter belief, but that's just me.