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The Tao trio is very creative and ingenious.
Reshiram and Zekrom represent truth and ideals respectively; the former looks natural and organic and therefore "real" while the latter looks mechanical and efficient and therefore "ideal"; Reshiram has a nuclear reactor for a tail wich is the most efficient energy source known today in real life and makes it a fire type, while Zekrom has a clean generator tail wich is the kind of ideal energy source we hope to achieve one day and makes it an electric type.
They were originally one and the same alien dragon (wich explains their biomechanical appearence) that got separated, and the empty shell that remained became Kyurem, wich is an ice type to represent absolute zero. This ice type negates its dragon weakness to ice attacks, while its dragon type makes it good against Reshiram and Zekrom (dragon types themselves) while neutral to fire and resistant to electric, making him a good foil to the duo.
