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 are a limited number of Pokémon that can be possible Raid Bosses. The availability of these Raid Bosses have been altered once; excluding the Legendary Pokémon.

In past events (regional ones specifically), there have been exclusive Raid Bosses that cannot normally be encountered at Gyms.

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.