It's probably because we've had a lot of frogs by now. Poliwag evolutionary line, Croagunk evolutionary line and Tympole evolutionary line for example.
And yet, Vulpix line and Zorua line are all foxes too, in the same way Chespin reminds me of Pikachu, Pachirisu and Emolga (I actually thought Chespin was the new Gen rodent, but then the other starters appeared).
It's not that they are unoriginal, but they're similar to other Pokemon from the same species. Just like the real world animals.
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