ANSWER - I like the idea of working with an animal in principle but although I can see the value of including an animal in your circle work I do wonder if the practicalities might make things a bit awkward for you.
The trouble with animals is that they don’t necessarily recognise circle boundaries, they are easily distracted by outside influences and they always pick exactly the wrong moment to do a foul smelling trump. And nothing kills the concentration at a serious moment more than a stench from your beloved pet’s back end…
So I think you need to consider the pet you use less from a magical perspective and more from a practical perspective. Magically each animal will have different attributes so unless you plan on focusing almost exclusively on one type of magic there will be times with any pet where they will add more value to your casting than others. A frog for example might bring a strong association with power and strength but would lack the intelligence of a cat or the adaptability of a snake.
The first thing I would think of if I were you is a pet that isn’t likely to wander off in the middle of what you are doing. I’d dismiss cats and dogs because of this factor. I know some witches who have managed to cultivate a cat for magical purposes but all five of mine have proven themselves to be utterly devoid of any ability to even sit still, let alone assist in my casting. Unless you are exceptionally lucky cats and dogs are tricky pets to work with.
Amphibians move very quickly so I think a frog or a toad would disturb your concentration because you’d be waiting all the time for it to hop off. Same with reptiles – though I’ve had pet snakes that seem happy enough to sit in one place so possibly you’d have some luck with a corn snake.
Rabbits – well they might work but I’m actually quite scared of bunnies so I don’t want to think too much about them (I watched Watership Down too often as a child!). Rodents – especially rats would work well because they are clever creatures who bond well with humans and who could be persuaded to curl up in a warm pocket and stay still. Again though it would depend on the rodent. And whatever you do, do not under any circumstances use a degu… Goddess alone knows what kind of forces would be released into the universe if those little madams were ever trusted with spell work!
I think my regular readers might know which animal I would personally recommend for circle work. As a keen tarantula enthusiast I don’t think you could go far wrong with including an eight legged friend in your magic. Granted you need to ensure you have a docile, slow moving terrestrial new world tarantula (something like the Chilian Rose) that won’t be tempted to either run off or give you a venomous bite. But the advantage of the tarantula is they stay still, they don’t make sudden movements, noises or smells and they have a menacing energy which can be very useful in hex work. If your primary consideration is a pet for spell work then you really can’t go wrong with a tarantula and you have the benefit that they are low maintenance, cheap to keep and really rather friendly as well.
Image – I’ve gone for a rabbit as a picture of a tarantula might make my readers jump whereas the bunny is only going to frighten me!) http://www.freeimages.com/photo/1195681donzeladef