Update: Niantic have confirmed that Deoxys Defense Form is coming to Pokemon Go as an EX raid battle on March 25. The leak stated it would come on March 23, but between the time of the leak and announcement this has clearly changed.
The most recent leak confirms that the psychic-type mythical Pokemon Deoxys, more specifically its Defense Forme, will made available as an exclusive raid boss on March 25.
Deoxys Normal Form and Attack Form have also previously been available in ex raid battles. These two came after Mewtwo was the first Pokemon to be available as an ex raid boss.
An ex raid battle is the same as traditional raid battles but it is invite only. It is also known as an exclusive raid battle.
The locations of gyms that host ex raid battles are random. To receive an invite to one a Pokemon Go trainer must have successfully completed a raid at the same gym.
Like Rayquaza, another Pokemon who has been leaked to appear a week before, Deoxys was first seen in Pokemon Ruby and Sapphire.
Deoxys due to its various forms provides some incredibly high stats in its specialized form. Naturally the Defense Forme variant of Deoxys has amazing defense stats.
Its stats are as follows:
Mewtwo, Tyranitar and Gengar are the best counters for Deoxys Defense Forme. Interestingly, you will need at least three other Pokemon Go trainers to take down the ultra-defensive Pokemon.