A datamine of Pokemon Go's most recent update provides a hint about the game's next Raid Boss. Recently, Pokemon Go updated its Game Master file, a collection of data that provides the game with various technical stats about attacks and Pokemon. Dataminers looking at the most recent update discovered that Darkrai, a Mythical Pokemon originally found in Pokemon Diamond and Pearl, was now a transferable Pokemon. If Darkrai can be transferred (either by a Pokemon trade or a professor) that means that players should be able to obtain more than one Darkrai, which likely means that the Pokemon will be available either as an EX Raid Boss or as Legendary Raid Boss sometime in the future.
In the main series games, Darkrai is a Mythical Pokemon with the ability to eat dreams. Players anticipate that Darkrai will be one of the best counters to Psychic-type Pokemon in the game due to its strong attack and its access to Shadow Ball, which can mow down most Psychic-type Pokemon in moments. Darkrai's only major weakness is that many Psychic-type Pokemon also have access to a Fighting-type attack, so it might get knocked out before doing too much damage.
Darkrai isn't the first Mythical Pokemon that can be transferred in Pokemon Go. As an EX Raid Boss, players could obtain and transfer multiple Deoxys as well. And while it would make more sense to make Darkrai a EX Raid Boss, it's also possible that Darkrai could replace Giratina Altered Form as a 5-Star Raid Boss just in time for Halloween.
Pokemon Go hasn't made any announcements or teases about Darkrai, but expect some news soon. If developers are tweaking Darkrai's stats in the Game Master File, that likely means that we'll soon have the chance to battle this Pokemon in some sort of raid.