Aggron is a huge, bipedal Pokémon. It is primarily gray with plates of silver armor. The armor on its head has two pairs of holes with horns protruding from the foremost holes. Its forehead plating extends past its upper jaw in a small point. It has sky-blue eyes, and two nostril-like holes on the tip of its upper jaw. On the rim of its mouth are several pointed, fang-like protrusions and the nape of its neck and back have gray-silver armor sections. It has wide, blunt spikes on its shoulders and gray-silver bands on its arms and legs. Its hands and feet each have three claws. It has a thick, long, black tail that it can swing at enemies.
Mega Aggron is bulkier and more heavily plated than its previous state. The horns on its forehead are now shorter, and it has a third, wider horn on its nose. Using its horns, it is said to be able to destroy a steel tank. A spike projects downward from both its upper and lower jaw. The protrusions on its shoulders are longer and curve toward its head, and new spikes thrust from holes in its armbands. Its tail is thicker with two bands around it.
The gouges in Aggron's armor from previous battles are worn as mementos. The more wounds it has, the more it has battled. It claims a large mountain as its territory and fiercely defends it from trespassers. If its environment is damaged by a flood or a forest fire, it will restore the area by bringing in topsoil and planting trees. It burrows through the strongest of bedrock and digs tunnels, as it searches for iron to eat. Its horns, which it uses to dig through bedrock when seeking food, grow a little longer at a time and can be used to determine its age. Aggron's horn is sharp enough to cut through steel and used as a ramming weapon to bring foes down. Long ago, there was a king who wore a helmet resembling an Aggron in an attempt to channel its strength.
Multiple Aggron appeared in Lucario and the Mystery of Mew, most of them as part of the feuding armies that were eventually placated by Sir Aaron in the past. In the present day, a Trainer's Aggron won against another Trainer's Hitmonlee during the Rota tournament.
Aggron is surprisingly protective of its environment. If its mountain is ravaged by a landslide or a fire, this Pokémon will haul topsoil to the area, plant trees, and beautifully restore its own territory.