Latios was originally revealed to be joining Pokémon UNITE along with Latias as part of the 4th Anniversary celebrations. Latias's release date is expected to be revealed soon.
Latios and Latias are both available to play on the public test server since June 16th. The Public Test app aims to gather data for balance changes and bug fixes, which will be utilized in future updates. The app is available only on Android devices in English in Japan, the Philippines, Indonesia, Thailand, Malaysia, Singapore, or India.