Ico was in development for about 4 years. In the first stages of development Ueda created an animation in Lightwave in order to get a feel for the final game and better convey his vision to the team. In this animation Yorda was the one to have horns rather than Ico and looked rather different in general, as can be observed in the video of Ico’s demo reel, in particular the scenes of Ico looking down on Yorda on the couch and Yorda’s hand examining Ico’s face. In the final game Ico was given horns instead in order to be more easily identified onscreen.
Originally planned for a release on the PlayStation, development started in earnest in 1998 with a team initially composed mostly of people from outside the game industry. Game design was guided by three key notions: to make a game that would be different from others in the genre; that would feature an aesthetic style that would be consistently artistic; and that would play out in an imaginary yet realistic setting. This was primarily achieved by subtracting common gameplay elements, such as the HUD, leaving only what is essential. Another method was using key frame animation instead of the more common motion capture technique.
Development switched to the PlayStation 2 in late 1999, which gave the team a more powerful platform to better achieve their original goals. Elements present around this time included a storyline trying to get Yorda back to her room, rather than an all-out escape from the castle and enemies that resembled the horned warriors seen at the beginning of the final game, rather than the shadowy wraiths. Despite these and some other differences, stylistically the game remained faithful to the original vision presented in the video, as well as always maintaining the core gameplay elements that center around puzzle solving. – [info from Wikipedia]
Ico PSX Version