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Reasons These Monsters Returned
(Source: Monster Hunter XX Production Notebook)

Nerscylla: Originally, Nerscylla was planned for Monster Hunter Generations, but there were problems in development that made it hard to put it in the game (In a sense, you could say Nerscylla broke the game). However, these problems were fixed in Monster Hunter Generations Ultimate.

Basarios and Gravios: Gravios and Basarios were brought back in Monster Hunter Generations Ultimate purely for their challenge factor. The MH Team felt that both were difficult enough to encourage new players to try different styles and arts on them if they faced problems while hunting them.

Giadrome: Giadrome was brought back in Monster Hunter Generations Ultimate for nostalgia reasons. For old fans, ones that started with Monster Hunter Freedom 2/Monster Hunter Freedom Unite, this was the first large monster they ever fought, and this was their first trial as a hunter like Yian Kut-Ku was for 1st Gen hunters.

Barroth: Barroth was brought back in Monster Hunter Generations Ultimate as a monster for beginners and longtime hunters of the series to hunt. The Monster Hunter Team had trouble integrating its muddy bogs into the game, and they had to change Barroth's model a bit as well to fit into Monster Hunter Generations Ultimate.

Barioth: Barioth was brought back as an obstacle for hunters to conquer in MHGU. Though Barioth is an easy monster to learn, the MH Team felt that it would be a fair challenge for hunters to overcome in the game. They also brought it back because they like its armor, especially for female hunters.

Diablos: Diablos was brought back as the gatekeeper to G-rank in MHGU, and because the MH Team wanted to give it a Deviant, Bloodbath Diablos.