The Eon Duo was introduced in a Sapphire, Ruby and Emerald special event. They can also be found in the Pokemon Black and White series. The Eon Duo is a fan term for the legendary male and female Eon Pokemon, Latias and Latios. Latias is the female Eon Pokemon. It's defense and special defense stats much higher than Latios's. However it has a signature move called Mist Ball which lowers the special attack stat, which is useful against Latios. Latios is the male Eon Pokemon. It's attack and special attack stats are higher than Latias's. It also has a signature move called Luster Purge which lowers the special defense stat, which is useful agains Latias. If you have both pokemon in your party, its a balanced lineup in a double battle. In Ruby and Emerald, Latias was chosen to roam Hoenn, while Latios in Sapphire was to do the same task. An Eon Ticket will take you to the Southern Island where you can encounter Latias or Latios depending on the version your playing. In Soul Silver, holding an Enigma Stone you can find and capture Latias in Pewter city, where you can do the same with Latios in Heart G
Latias and Latios

Latias (Left) and Latios (Right) The Eon Dou