Fumito Ueda, the creator of Shadow of the Colossus, originally had 48 colossi in mind when planning the game. Realising that putting in that many colossi was not possible, he cut the number down to 24 - half the original number.
Eventually, the number of colossi used in the game became 16, with the 8 unused colossi appearing only in the artbook. Some are quasi-prototype versions of colossi that survived the final version of the game, other designs were discarded to improve game flow and some were cut for reasons unknown.
It is a possiblility that all 32 or at least, 8 of the unused colossi might be put back into the 2018 remake of Shadow of the Colossus.
Known Unused Colossi
|Developers code name||Name given by fans||Resembles|
|Roc||Avus||Bird of prey|
Interview with the game the director and project manager, from the official guidebook and artbook.