Yveltal's appearance is based on an avian in the shape of the letter Y, and as such has many similarities to the long-tailed widowbird. It may be based on any number of figures from mythology, such as the Celtic death-goddess Morrigan, which often takes the form of a raven, or Norse mythological figures such as Hræsvelgr, which takes the form of an eagle and sits at the end of the world, or the Hawk Veðrfölnir or the unnamed eagle that resides at the top of the world tree Yggdrasil. Yveltal also may draw inspiration from carrion birds such as crows and vultures. Additionally, it bears a striking resemblance to a species of fungus known as "devil's fingers."
source tldr maybe
Yveltal looks like the letter "Y" also Yveltal aren't based on common birds in the real world it should be based on a mythical creature
List of flying mythological creatures