Have you ever done an entry on the Eevee evolutions? The difference I think is that they would be considered a domestic breed versus a wild species. Eevee changes with the stones of water, fire, and thunder. They also change with friendship during morning(Espeon) and night(Umbreon). Finally, they are able to evolve due to rocks, moss covered for Leafeon and ice covered for Glaceon.
Actually it's more simple than one my think. I hope this doesn't remove the mystique but its a stock parchment image that I layer underneath all the artwork. I also reduce the opacity on all of my paints to give each piece a watercolor feel. I wanted this to feel as if the Professor was in the field making observations and taking notes to later put into a Pokedex.