Raid Boss

A Raid Boss or Boss Pokémon is an extremely powerful Pokémon that has very high CP. It hatches from a Raid Egg which appears atop a Gym upon the beginning of the Raid Battle. A countdown will display the time until the Egg hatches and the battle begins.

Details
Upon using a Raid Pass to join the battle, the Trainer and up to 19 other Trainers work together to defeat the Raid Boss. If a Raid Boss is successfully defeated within the three-minute time limit, those Trainers have the chance to catch a decently powerful Pokémon of their own.

A Raid Boss can battled as many times before it is either defeated or the one-hour time limit expires.

List of Raid Bosses
There is certain set of Pokémon that can be Raid Bosses. There was currently one change in this set.

During some events, specifically, there are exclusively some Pokémon as Raid Bosses that regularly are not Raid Bosses.

Trivia

 * The Raid Boss icon is based off of Rhydon's face — who was the first Pokémon designed and programmed into the original game. There are also various Rhydon statutes that can be found throughout the game as well.
 * On November 4, 2017, the raid bosses were rotated for the first time.