From what is seen from early promotion videos of the game uncovered, there is a lot of evidence to suggest that Basaran (the turtle Colossus) and Quadratus (the mammoth) were the first Colossi to be programmed into the game. Of the Colossi shown in preview versions, these two were the only ones to have proper battle footage, while most others were mere cutscenes.
Additionally, thanks to the many leaked preview versions of the game that have been dug up, there's a lot of evidence to suggest that Cenobia was the last Colossi to be programmed into the game, as they are missing in every single version of it sans the final released version.
Based on the 2005 beta-ending had the Roc's tail as part of Dormin's design and the fact that it is currently the only beta-Colossus to have actual gameplay footage, there's heavy evidence that the Roc's removal was a very late decision; likely the last of the beta-Colossi to be removed from the game.
Tropes
Colbert Bump: Had a bit of one when it was used for Reign Over Me. Adam Sandler was a fan of the game when it was demoed to him and requested it be put in the scene where his character plays video games to cope with the loss of his family, mirroring Colossus' plot. The scene where he and Don Cheadle played through it wasn't scripted, but footage of them playing the game naturally and being immersed into the experience.
Development Gag: The game selection screen for The ICO & Shadow of the Colossus Collection has Shadow represented by the abstract artwork that was going to be the cover back when it was going to be called NICO.
Doing It for the Art: Though all of Team Ico's oeuvre qualifies, this one really stands out for its blatant defiance of many conventional game mechanics. Minimal replay value? A string of sixteen boss battles with no action connecting them? An extremelyminimalist set-up, with basically no game mechanics apart from a sword, a bow and a horse? A grand total of four characters with hardly any dialogue between? What was Team Ico thinking, exactly? Simple: they weren't out to make a blockbuster action game that could spawn tons of sequels, they were out to make an interactive art epic that could tell a story through its gameplay and its visuals. If the game's unanimous critical acclaim is any indication, they succeeded. And if the game's commercial success is anything to go by, yes people will pay money for these kind of games.
The "dam". No one knows why it is there or what it would have been for. It does not even have collision data (meaning you cannot stand on it), and it is only accessible through hacking and a lot of patience. Someone did manage to add collision, giving a better idea of the sheer size of the area.
The entire east section of the map will always be covered by clouds. There is a bridge over the 12th Colossus area that could have been an access to it, but there is no way to get up there.
The original plan was to have a whopping 48 Colossi in the game, though this was cut down to 24. Another eight of these were then removed from the final product for pacing; they were either combined with the one or two of the remaining Colossi or just plain scrapped.
In Memoriam: The 2018 remake is dedicated to Susan Benedict (Bluepoint Games's studio manager) who died three years before the game's release.
The fabled 17th Colossus. Same for the upper levels of the temple, above the secret garden. However, hacking the game can lead to some very interesting finds.
As to just what the 17th Colossus is, there are a few interpretations. One says it's a boar-like creaturenote possibly based on the beta-boar Colossus that got removed that was encounter in the secret garden after completing a certain event. Exactly what the event is varies, but supposedly, the way to beat it was to trick it into the hole that Dormin communicates to Wander through.
Another interpretation of a 17th Colossus was a giant sea serpent. It was supposedly encountered by swimming far off into the ocean around the Forbidden Lands in any direction. How it was beaten is unclear, but supposedly, either it would act as Border Patrol and auto-kill Wander or it was a legitimate battle, and/or it was supposedly larger than any of the other Colossi in the game.
Yet another popular interpretation is that the 17th Colossus is supposedly a dragon. Like a good portion of the 17th Colossus that couldn't possibly fit into the secret garden, it was supposedly above Pelagia's lair, across the bridge that extends high above it.note Said bridge is actually impossible to reach without hacking Notably, this interpretation became far more popular after a really well put together fan animation tribute was posted to YouTube.
There were tons of supposed ways you could supposedly win the Hopeless Boss Fight against Emon and his men posted online, which would lead to an alternate ending to the game. Naturally, none of these were true. There were also rumors that if you successfully reached Mono during the Controllable Helplessness segment, it would lead to an alternate ending. While in the original PS2 version, it's possible to abuse a glitch/physics exploit to reach the altar, all Wander is presented with is an Invisible Wall just before the stairs, although it does reveal a funny oversight that was fixed in the later versions where Mono's body isn't loaded in during that scene.
Word of Dante: The Fan Nicknames for the Colossi (Valus, Avion, Celosia, ect) are widely believed to have originated from a GameFAQs walkthrough, which the fandom fully assumed to be official.
◊, including the locations of the cut Colossi. A Mega upload of everything that was leaked
