It's actually about "ancestral bloodline manifestation" (IDK the correct translated term that means 返祖現象, maybe atavism?) that passed down in their royal genes. Olivia is not a human too due to the same reason... Ain's identity is even more complicated and only a half-human that's not even birthed through human's reproductive method but that's another thing already, which if things go on like what the story and dialogues are implying in the story, actually makes his possible relationship development with his mom more complicated and potentially problematic/controversial for some readers despite it's in a fantasy world.
Basically, in their fantasy setting, some humans might revert back to the species that some of their ancestors were. So technically, Kadyma the 1st princess isn't a "half-cat" offspring of a human and a cat, but a beastfolk/beastkin/beastman, or maybe a human with ancestral's species abilities and physical appearance, it's not explained in-depth yet up to the point at where I read in the novel.