Your problem can be solved canonically one of two ways:
The joys of the limitations in hermetic theory. My current character in a game was dating a Magic Human that developped the capacity to shapeshift into drake form over time. Part of the theme for the storyline of the past few years was "I can't make you my familiar, sadly. Let me increase my magic theory, then I can look for a practical solution to that problem." At one point, as we were getting close to starting the original research, the solution came from another route: she underwent a process of transformation as a result of a story, becoming a drake that can change into human form and the virtue changed from Magic Human to Magic Animal and suddenly I was wondering why I was looking at years of original research when the only stat change on paper was the name of a virtue and which form was the natural form prior to the shapeshifting.
I've had a similar feeling about some other stuff. I'm not so worried about Magic Humans. But, for instance, there is the homunculus. I'd be happy having elementalists bind elementals, too. Of course, spirits do require one of several special Virtues. So at some point requiring a discovery should be reasonable.
Perhaps I should tweak the origin story to:
Horse heartbeast Bjornaer sires colts while in horse form. Later when Bj mage becomes Great Beast a foal descendent being born at that moment is born with a corpus monstrosity human torso and head, representing the humanity lost by the Bjornaer
The way I see it, any thong (or one) aligned with a realm can come in a faerie flavour. Whether it is a unicorn (divine or faerie), mythological creatures (magic or faerie), as the story about the original is likely to attract a faerie who will try to inhabit the personnage for he vitality. While I don't know how long a faerie pretending to be a demon might take before it switches into an infeenal being, Krampus (or anachronistically a Hans Trapp) could be aligned with either realm.