When Yveltal's arms itself in flight, it looks like the letter "Y".
Yveltal is the fourth Pokémon to start with a Y, the others being Yanma, Yanmega and Yamask and the first Legendary to do so.
Yveltal is opposing Xerneus due to Xerneus representing life and Yveltal representing destruction.
Its form may be a reference to Norse mythology guardians of the Yggdrasil Tree: a bird-like creature that protects the top of the tree, this may be Yveltal, a deer-like creature that protects the soil and trunk of the tree, this may be Xerneas, and a snake-like creature that protects the underworld below the tree. If this is right then there might be a snake-like legendary Pokémon to go with Xerneas and Yveltal. It may also be based off a wyvern.
Its name may come from the word tall since it can fly and start with the letter Y because it represents the Y axis in the 3D axis of X, Y and Z.