Niantic has announced that the Mythical Pokemon Deoxys will replace Mewtwo in EX raids.
In a post on Pokemon Go’s official site, Niantic confirmed that Deoxys’ Normal Forme (one of four possible forms of Deoxys) will only be accessible via EX Raids, which require an invitation in order to participate.
However, starting with Pokemon Go’s newest update, players will be able to share an EX Raid with one player on their friends list, as long as they’ve achieved Ultra Friend status or higher.
Source: IGN.com Pokemon Go: Deoxys Confirmed for EX Raid Battles