is it normal to realise a kintype long after you’ve already seen/played/etc the source material? that seems to happen a lot to me, i even have one kintype that i only realised after reading a fanfiction about them?

It can happen, though I have no idea how common or uncommon it is. Awakening a  kintype starts when something triggers feelings of a connection/memories. Maybe you’re more closely related to this fanfiction than the ‘canon’ material presented in this universe.