I could be wrong, someone please correct me if so.
I think the first series or so released all the models in roughly equal amounts. Then they started making certain models in lesser quantities. To some extant I think this reflected the classification of the species in real life (the more endangered it was the fewer figures there were). But I am unsure on the real numbers. The glass frog in particular, as well as a skateboarding celebrity bull dog, and the new giraffe calf of the babies series are "chase pieces".
I will note that certain regions of the country seem to get different frequencies of models too. For me something I found rarely in the Pacific Northwest region of the US was common to say, Kelly in the Southeast region, and vice versa.
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-"I loathe people who keep dogs. They are cowards who haven’t got the guts to bite people themselves."-August Strindberg (However, anyone who knows me knows I love dogs
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-“We can try to kill all that is native, string it up by its hind legs for all to see, but spirit howls and wildness endures.”-Anonymous