https://www.polygon.com/2017/11/2/16598408/pokemon-go-new-pokemon-unavailable-legendaries-gen-2

A Pokémon Go dataminer has found models of unreleased Pokémon inside the game’s latest update. These assets belong to two of players’ most anticipated monsters — leading some to speculate that Niantic will soon make them obtainable.

ZeChrales, a member of the Pokémon Go community who regularly dumps assets on their GitHub page, worked through the game’s latest version and hit on fully-rendered models of Ho-Oh and Celebi. These are two of the remaining second-gen Pokémon as of yet unobtainable in the mobile title, and the only legendary monsters from that generation that players can’t yet capture.