Aggron is best described as a monster. It bears a dragon like appearance with iron plates over its entire body, horns, and claws; this Pokémon was built for battle. Some say that it looks partially like a castle wall.
Aggron evolves from Lairon at level 42, and is the final evolution of Aron. It can mega evolve into Mega Aggron.
Aggron plays an important role in Pokémon Conquest, as the Partner of Ieyasu, the Warload of Valora, with a +2 Iron Head.
"Aggron Mega Evolves into Mega Aggron. When it does, the destructive ability of its fearsome horns can turn a steel-clad tank into so much scrap metal. Aggronite, the Mega Stone needed to Mega Evolve Aggron, can only be obtained in Pokémon Y. By Mega Evolving, Aggron changes both its type and its Ability. By changing to a purely Steel-type Pokémon, it reduces its weaknesses, and its Filter Ability also reduces the damage it takes from supereffective moves." 
Its name is probably composed of the words "aggressive" or "aggravate" and "iron".