Latios appears to be a cross between a jet plane and a dragon. It and Latias may also draw inspiration from the philosophy of yin and yang. In addition, they may be based on the aeons in Gnosticism, being that certain aeons are immaterial beings of concepts existing in pairs of males and females emanated from God and one another; also, their category is "Eon". Latias and Latios are also based on birds, and due to Latias's shared origin with Blaziken, they could be based on junglefowl.
Source
So they're a duo and that paragraph explains why they're the Eon Pokemon (note that they're both categorized as the Eon Pokemon, so that's why their categorization is referenced in the duo's name).
Hope this helps! :)
