"An insightful question, Miss Potter," he says, with all the delight of a career academic being asked about a pet subject. "It's difficult to be sure, but it seems two separate shapeshifting techniques actually made their way over - one Nordic, one recently Germanic, though that one had perhaps yet more foreign roots. The British-Nordic tradition eventually died out except in the form of shapeshifting curses - one descendant of which would eventually create werewolves, though that's a bit of a digression. The British-Germanic tradition, however, survived. It developed two main branches - that of the Irish Druids, and that of the British Hedge-Witches. The druidic tradition was always a secretive one, and sadly their shapeshifting tradition was among those they lost in one of their teaching lineage disruptions - but the witchcraft tradition remained strong, spreading easily. An early medieval coven actually required initiates learn to change shape before they could be considered full adults - a line often drawn between eleven and fifteen, in those times."
"The witchcraft tradition, of course, met wizardry, as those magicals who practiced in the Roman tradition slowly came to an accord with the older British magicals. The founding of Hogwarts provided the true catalyst for the melding of the two, however, for our esteemed Founders were determined to teach both. Salazar Slytherin was the most famous witch of his age - the only one of the Founders able to change shape - and one of his students, Amice Becca, modified the witch transformation with elements of self-transfiguration, creating what was very nearly our modern Animagus technique - modulo a few tweaks down the centuries."
"There had been existing wizarding shapeshifting traditions, of course - but all of them were immensely difficult to master, requiring a new spell with a new risk of error on each change. The witchcraft traditions had been far easier to learn, but Lady Amice believed that they had a clear drawback - it was rather easy to reveal a witch in animal form and to trap them in one form or another. The Animagus ritual had none of these problems. It was still incredibly difficult to learn, especially for those not educated in both witchcraft and wizardry, but an Animagus cannot be prevented from transforming directly, though you may trap them in an environment hostile to one of their forms, and once you complete the Animagus ritual, you retain the ability throughout your life, regardless of whether you have a wand on you. And, for centuries, Animagi could not be revealed or forced to change - the Animagus Revelation spell was only invented in the nineteenth century, and the Animagus Reversal spell, which can return an Animagus to their human form, was invented shortly after the turn of the twentieth."
"Of course, nowadays the witchcraft traditions largely lack the drawbacks they used to have, as widespread knowledge of witchcraft outside of Potions and Herbology has waned. Those old traditions are mostly a concern of dusty old men like myself."
He pauses.
"I rather rambled there, didn't I?" he says, a bit sheepish.