This is also a type of Golem. This is a creature that goes along with a witch, or a warlock, and most wizards. It is a base to help them with their magic. Depending on the creature they choose, depends the nature of the magic that is brought forth. It's not purely physical attributes, yet mental ones as well.
Rats are common, as they are always energetic, and quite greedy.
Snakes are lazy, yet tranquil and can stand to be in closed, lonely spaces for long times. This patience could be a good attribute.
Cats are mostly lazy, yet playful when they need to be. They aren't one to take orders, yet they do what they need. They are resourceful indeed.
Birds are considered to be wise, as they too are logical creatures. Owls especially are a good choice, as they have great sight, and don't do much unless it's time to feed. Also, the nature of their strange movements might be a small factor on some aspects of magic. Ravens as well are common.
Fish have been chosen before, but I would seriously not know why. They are entirely stupid, perhaps. They have little or no memory span. They have the ability to breathe underwater...
Frogs have been chosen for their ways as well. Amphibeous beings, able to live both in water and on the Earth. They aren't too bright, yet their being and nature of slight intelligence, as they wouldn't fear you if they knew you, like how a fish would rarely, -rarely- let you touch it.
All types of creatures can be used, yet be warned with the magic they do become moreso intelligent and sentient. This could be potentially dangerous, as they can steal a wizards power, and become the wizard. Rare, though.
If you are a familiar though, go for it...If you get turned into ash for trying, don't blame me.