The Soul Wanderers

"Students may also bring, if they desire, a owl or a cat or a toad." (so said Harry Potter's acceptance letter to Hogwarts)
Yes, I did just quote Harry Potter, but not for no reason. The actual quote of wisdom is this:

From Manly P. Hall's The Secret Teachings of All Ages (1928):
"Certain plants, minerals, and animals have been sacred among all the nations of the earth because of their peculiar sensitiveness to the astral fire--a mysterious agency in Nature which the scientific world has contacted through its manifestation as electricity and magnetism...The magicians of the Middle Ages surrounded themselves with such creatures as bats, spiders, cats, snakes, and monkeys, because they were able to appropriate the life forces of these species and use them to the attainment of their own ends."

Cats, snakes, bats, owls, monkeys...they are all "magic" animals, and often seen in pictures with witches or magicians. This is not just a coincidence: it is due to the "astral light" (same thing as astral fire) that they draw near them. This is also known as animal magnetism, which was named by Franz Mesmer in the 1700s for a natural invisible force exerted by animals. The astral light, named by Medieval mystics, is  mystical light from a higher world, commonly associated with fire and the Sun. It is a connection between the higher worlds and our lower world. The Theosophical Society says that "the astral light or astral plane is another world, or another story of the world, related to a different set of human senses, and characterized by different relations of space, time, and other qualities." It is also what Eliphas Levi calls the "universal imagination" that we can all access and gain inspiration from. However, it is not simple to access this astral light, and that is where these special animals come in.
Supposedly, certain animals such as those mentioned above have a connection to the astral light. Why a cat and not a dog? I have no idea. It may be that it just happens naturally, so that it doesn't make sense to ask the question (like asking "why do chickens cluck instead of bark?" Well, it's just the physiology of a chicken that makes it cluck).
So magicians would keep these animals near them so as to borrow the powers of the astral light for their magical endeavours. The animals weren't necessarily conscious of their connection to this power, but emanated the astral light naturally. They did not have to harm the animals to do so, for the astral light is not a physical substance that can be attained by some magic potion. However, we know that witches' brews often contain animal parts, but from what I've read, it seems that the true magicians do not need to physically concoct something in order to use magic. Just being near creatures with this astral magic can help bring more of it into their being and so allow them to be more attuned to the higher worlds. This might be another difference between "light" and "dark" magicians.
Cats are especially potent in the astral light, and the ancient Egyptians certainly knew this, keeping cats in their temples. Serpents too were kept in temples in Ancient Greece, such as with the Oracle of Delphi. The serpent was also a symbol of wisdom, and perhaps this is due to its ability to attract the astral fire.
So Hogwarts students carry on the tradition of Medieval magicians with their magically potent animals, and maybe you too, if you have a pet cat.



You can learn more about the astral light here.
Manly P. Hall's book is here. It's very long, but very worth the read!