The first interactive game to feature a boss was dnd, a 1975 computer role-playing game for the PLATO system. One of the earliest dungeon crawls, dnd implemented many of the core concepts behind Dungeons & Dragons. The objective of the game is to retrieve an "Orb" from the bottommost dungeon. The orb is kept in a treasure room guarded by a high-level enemy named the Gold Dragon. Only by defeating the Dragon can the player claim the orb, complete the game, and be eligible to appear on the high score list.
The first arcade game to feature a boss was Phoenix, a fixed shooter developed by Taito in 1980. Phoenix includes five levels ("Rounds") which pit the player against swarms of alien birds. During the first two Rounds, the player is assaulted by the pigeon-like "Scouts", whereas the more formidable "Soldiers" are introduced in Rounds 3 and 4. On disposing these enemies, a giant mothership appears in the fifth and final Round.
A "boss rush" is a round in which boss characters from earlier in the game are fought in succession, often leading up to a big boss. This is found in such games as Bonk's Adventure, Crash Team Racing, Space Harrier, Monster in My Pocket, Mega Man, and the Kirby series.