A familiar can be any animal that is close to you, but remember, they choose to be your familiar not the other way around. And as the name suggest they must be familiar to you. This can be a family pet or a stray animal that follows you around, or even a wild creature that you are particularly close to.
This animal may sit with you or be nearby during your workings, protecting or aiding you and lending themselves to your purpose.
A friend of mine's familiar, a lively dog, ran off while out for a walk. Upon it's return it was carrying a very large branch that it dropped at his feet. It showed no interest in playing fetch only giving t stick up. When my friend finally looked at it he realised that it was charged. After thanking the dog he turned the stick into a staff that served him well for many years.
My cousin has a rook that follows her around. He is an old bird and noisy but is relaxed and silent when she is working. He stayed nearby during our last ritual together and only cawed to keep away anything that would interrupt us.
Remember, take care of your familiar and it will care for you.