He's awesome.
Each member of the evolutionary line combines an animal with the chestnut shell theme. Chespin is kinda like a porcupine, Quilladin is straight-up hedgehog. Both have spines, like a chestnut, though they lack a "shell" its still a cool design with a fun fantasy element.
Chesnaught seems to have a lot in common with a group of extinct animals called glyptodonts. Glyptodonts were relatives of armadillos/anteaters/sloths that lived in South America (and eventually North America) and went extinct toward the end of the most recent Ice Age. They had a large domed shell of bony armor, as well as a helmet-like plate on the head and armored tails. Sound familiar? Chesnaught has a lot in common with these creatures, even though GameFreak made some changes like turning him into a bipedal bruiser instead of a trundling tank. Some species even had spines on their tails, and their front limbs had sharp claws (like most of their relatives) with flatter hind digits, also similar to Chesnaught.
As for the color change from green to a lighter tan/cream, chestnuts also brown and fade with age, so it may have been a design choice to make Chesnaught look like an aged, grizzled warrior.
