Beginning March 25, 2019, Defense Forme Deoxys will appear as an EX Raid boss available for Trainers packing EX Raid Passes. Deoxys itself is a Mythical Pokémon, but it is has several different forms. Defense Forme is one such form and packs much greater defensive capability than its Normal Forme.
In order to be eligible for an EX Raid, you’ll need an EX Raid Pass, which you have a chance at receiving after clearing a Raid Battle at a qualifying Gym. The EX Raid Pass lists the time and date of the EX Raid event and you’ll need to be there at the given time in order to participate.