Chesnaught is a bipedal, mammalian Pokémon with some plant-like features. There is fluffy white fur on its belly and fringing its face like a beard. It has a small, pink nose with a three-pointed patch of brown fur extending up its short snout, and four pointed teeth. On top of its head is a plate of beige armor, similar to a helmet. It possesses a prominent domed, armor-like shell similar to that of a chestnut. The armor is beige with four large spines jutting out of the back. The armor is trimmed with red and green bands; the green is on the outermost edge. There is an additional spine protruding from the armor over each of Chesnaught's shoulders, which are connected by a green breastplate. Its body is colored similarly to the shell, and its long limbs and tail are banded with plates of armor. The top surfaces of the tail and arms are protected by green armor, and each arm has two small spines. It has three brown, clawed digits on its hands and feet. Its hands can fuse together to create a spike-covered "shield".
Chesnaught is renowned for its tank-like body; it is capable of flipping a 50-ton truck. Once it gets into its defensive stance with its hands covering its face, it can stand up to a bomb's explosion. It will not hesitate to use its sturdy body to shield its allies from danger. In the past, Spiky Shield was its signature move.
Chesnaught has the highest base Defense stat of all fully evolved starter Pokémon.
With a total of six, Chesnaught has the most weaknesses of any fully evolved starter Pokémon.
Chesnaught seems to be based on a hedgehog combined with a matured chestnut. It may also draw inspiration from "armored" mammals such as pangolins, armadillos, and glyptodonts. Its bulky "armor" is possibly a reference to split apart calybium and cupule, which are parts that protect the nuts of chestnut trees which split apart when sufficiently ripened. It is also based on knights and various types of body armor, including brigandines, barbutes and modern bomb suits. Chesnaught and its pre-evolutions may also draw inspiration from the warrior class as most warriors specialize in physical combat and make great use of the most powerful heavy armor while having a much more chivalric aspect.