Do you mean a familiar, a spirit animal, a totem, or a spirit guide? These and other beliefs, even fictional (perhaps I should say misconceptions to be more accurate) have been confused by a lot of New Age teachings.
A familiar spirit does not necessarily have the form of an animal. Whether the practitioner believes it is called or created, it is a spirit which works along with the magician to accomplish works -- as a help and a follower; it performs tasks as instructed.
A spirit animal is a rather vague concept, especially these days. It can simply be an animal with which a person resonates for a time, or for life, with some corresponding qualities the person possesses or wishes to possess in their own traits. While some believe a spirit animal also plays the role of a guide, this is not always the case.
A spirit guide is not always perceived in the form of an animal, though it can be. It is what its name implies: It guides, sometimes protects, and often advises the practitioner. It can be a help in the astral, though often it is more a nudge to the intuition.
A totem is specifically related to Native American cultures, and varies greatly depending on which specific Native American group or nation's beliefs are being followed. Some nations simply had an animal as a symbol of their family unit, while some others found intense spiritual connections with their own as a group, and some as individuals.
One misconception about familiars is that people for whatever reason insist they can be physical animals. A familiar is a spirit. If you believe that a familiar is created (as in a thought form such as a servitor or tulpa), then yes: you can make it appear however in your mind you choose.