https://www.polygon.com/2018/9/19/17880504/pokemon-go-deoxys-how-to-get-ex-raid

Pokémon Go’s newest EX Raid boss is Deoxys, one of the rarest Pokémon in the franchise. A mythical monster from Hoenn, Deoxys remained one of the few remaining third-generation Pokémon not in the game — making its entrance a big deal for players.

The psychic-type Pokémon from Ruby and Sapphire is able to shift between multiple forms, but only its basic one, or “Normal Forme,” is available in-game for now. The only way to battle Deoxys in any fashion is to receive an invitation to an EX Raid, however, so unless you’re one of the lucky ones, you may not be seeing any form of it at all.