More Aggrons? More Aggrons. I have had this idea for a long time, years probably, and I finally got around to do it. This is also a very, very late attempt to jump on the Pokémon Variations bandwagon.
Shiny Aggron's green coloration inspired me to think about the reason for the particular green tone and it reminded me of oxidized copper. My headcanon is that Aggron eat minerals and metals to grow and keep their armor in shape. Depending on the Pokémon's habitat there may be differences in minerals available which result in regional differences in both appearance and properties of Aggron's armor. This particular Aggron - or Shiny Aggron 2.0 - has copper armor that gradually oxidizes. The younger the individual the shinier the copper, and the older the Pokémon gets the darker it turns and provided that the conditions are right it eventually turns green.
The "ancient" classification is misleading and mostly refers to the rarity of these fully oxidized individuals. In order for this to happen the air pollution must be minimal, the minerals pure and plentiful and the Pokémon needs to be healthy. The "ancient" classification may also be a reference to many important monuments that have survived time and turned green.
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Art © ShadeofShinon
Copper Aggron: variant of Aggron
Copper shell wyvern
"A variant of Aggron that has eaten large amounts of copper, to the point where it's shell is made of it. It's armor darkens with age.
Ancient Aggron: Rare species of Aggron
Oxidized Metal Wyvern
"A rare species of Aggron that lives in an area with vast quantities of pure minerals and minimal air pollution. The pure metal that it eats has made it's shell tough and green"