Design inspiration. Groups all the cat Pokemon (like Meowth, Litten, Skitty, ...) together, the fish Pokemon (Arrokuda, Wishiwashi, Magikarp, ...), inanimate objects (Klefki, Vanillish, Stonjourner, ...), and so on.
Could separate by location found. Divides into City, Forest, Water, Cave, Desert, Grassy, and Snowy fairly well. Considers Zubats synonymous with caves, for example.
Might be more of a relic of older times, but perhaps Diurnal, Nocturnal, Crepuscular (twilight), and the other time periods? Stumbles upon far more Hoothoots and Ghost-types at night.