Alpha-sapphire: Entei embodies the passion of magma. This Pokémon is thought to have been born in the eruption of a volcano. It sends up massive bursts of fire that utterly consume all that they touch.Y: It is said that when it roars, a volcano erupts somewhere around the globe.X: A Pokémon that races across the land. It is said that one is born every time a new volcano appears.
Prof. Lotus' Notes: During the events of the Ecruteak City, it is believed one of the mystery Pokemon that perished that night was a Flareon, and because of Eevee's unstable DNA, it could explain how Ho-oh was able to not only resurrect but change this Pokemon.
Entei's name is a possible portmanteau of the Japanese words 炎 (en / honō flame) 炎天 (enten - a blazing heat) and 皇帝 (kōtei - emperor), or the Kanji and Chinese reading of 炎帝 (honō ten - flame emperor in Japanese and Yan Emperor in Chinese, who was known as a Flame Emperor in pre-dynastic period)
Entei’s design is influence by Chinese guardian Lion / Foo Dogs. Lions are also considered King’s of the Jungle, which further strengthens his emperor design and status. That and the gold crown design that it has.