Eh, I don't think its that Nintendo/Gamefreak doesn't want mammals on the cover, its just whatever legendary duo they decide for that generation. However, it would be pretty cool to see some different Pokemon on the game covers, for example the starters? At the same time, it does make sense to show the legendary duo on the box art, so people will be aware of which legendary is exclusive to that game. (Also, could someone explain to me why they had Lugia for silver, Ho-Oh for gold, and then SUICUNE for Crystal? Made no sense to me why they would use a Pokemon from the legendary trio for the third version. Thats like if they did Dialga, Palkia, and then Mesprit .__.)