A witch's familiar is defined as an animal, usually a cat, toad, bird, mouse or dog that serves its witch as a loyal domestic servant, spy, companion or muse in witchery. I'm not sure I believe in the idea of the familiar as an acting agent for a witch, but I certainly do agree with them being companions and even muses. While not much of a cat person, I am very much a dog person and have always since childhood been drawn most strongly to black dogs, which may be my own personal version of the "witch's black cat."
My two dogs are my constant companions and we have a keen sense of communication between us that amazes most people who relate to their animals only as lowly animals, rather than as the uniquely sensitive and responsive creatures they really are. My dogs (and occasionally my cat) join me in circle quite frequently, sometimes getting cozy within its boundary for the duration of my ritual and sometimes coming and going quietly throughout. The interesting thing is that their normally silly, hyper behavior is instantly turned calm and respectful when they are within my sacred circle, even when my own energy is charged up in spellwork. And as to being my muses, I couldn't work without them. They are always on the floor at my feet as I paint in my studio or sit at my computer writing, and nothing calms my restless soul at the end of the day more than curling up with the two of them under the blankets as the night grows old (much to my husband's frequent chagrin).
I draw energy and inspiration from them. They make me feel alive, whether we're sitting indoors hard at work or romping through nature and feeling the beauty of Mother Earth. They speak to my inner child and the one that sits in perfect view for all to see. So while I may not call them my familiars, in reality that it what they are: the perfect unconditionally supportive and endlessly loyal companions who bear witness to all I am and all I do, both the magickal and the mundane, as I travel this path of my life. And it doesn't get more "familiar" than that.