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I know a few other people have pointed it out, and I don't think it's really meant to be a part of "muh prehistoric patterns" for Grass-starters, but damn, Chesnaught really did turn out looking a lot like a Glyptodon.
Starts off as a little porcupine-like guy (the buck teeth and rodent like face but with quills), becomes a bit more like his hedgehog namesake (pointed snout, more roly-poly appearance), and now he gets the domed shell, armored skullcap, and even the plated tail (which unfortunately can't be seen in this shot).
Considering that the whole idea behind the chestnut theme is "spiny shell", I think it's a bit interesting that the first two stages were based on animals with spines but not so much of a shell, with the final form being based on a shelled creature. If only Chesnaught had gotten a spiked tail-club conker like Doedicurus (another kind of glyptodont).