Groudon is a massive, bipedal, dinosaur-like Pokémon covered in red, segmented plates of thick, lava-resistant skin that act as armor. It has a gray underside and large white spikes that run along the sides of its head, body, and tail. Groudon has four claws on each hand, four dozer-like blades on the end of its large tail, and three claws on each foot with gray markings under its feet. It also has blue stripes inside the seams of its armor plating which can only be seen when Groudon is glowing with power.

Groudon is believed to be the personification of the land itself, and is said to be the creator of the world's continents. Groudon is an immensely powerful Pokémon, having the ability to summon intense droughts and cause volcanic eruptions. The droughts it can cause allow it to evaporate water, while the volcanic eruptions it can summon enable it to create land. Groudon was hailed as a savior for evaporating flooded areas of the world. Groudon can travel on water by using the magma to form a path. It sleeps underground in large magma chambers. If it encounters its rival, Kyogre, they will engage in a cataclysmic battle. Groudon loves Groudon Treats made by Snacksworth, who once encountered and waved to it in the past.

Forms
Groudon can undergo Primal Reversion and become Primal Groudon.

Primal Groudon's appearance is similar to Groudon, but larger in size. Its skin color changes from dark red to deep crimson with its underside, spikes, and teeth turning black. The blades on its tail are more separated from one another. The Ω symbol is now shown on its arms. Its eyes, the inside of its mouth, and the now-enlarged openings between its armor plating glow bright yellow, and its irises are now red.

The primal form of Groudon is said to be Groudon's true and original form. Groudon is at its full power when in the Primal state. The energy that fills Primal Groudon pours forth as magma, and it burns with such an extreme temperature that its body is always shimmering with the haze of its heat. In the anime, Primal Groudon is said to create new land with every step it takes. Primal Groudon was created by the natural primal energy deep inside the earth during Hoenn's ancient past. It feeds on the primal energy to maintain its primal form. Primal Groudon along with Primal Kyogre have the power to disrupt the planet's balance, with the risk of it being destroyed.

(Source: Bulbapedia)