I had this problem and then I came up with idea.
I am a new user and seeing how rare are the creatures from older years I'm pretty sure I'll have hard time getting any of them. Plus I think it must be hard coming up with new ideas every month. So why not put from time to time some old stuff for the monthly adoptnion. New users won't feel bad beacuse they can't get the older creatures.
Sorry if anyone allready talked about it, I don't feel like going through all those posts