The UNSC Strike Fighter was planned to be used as a heavy fighter aircraft for the player to use in the opening stages of Halo 2 but is was never added. Not much is known about this craft, except a brief description and concept art found on the Collector's Edition DVD.
Marcus Lehto: "I think it was the Strike Fighter, right? It was like a defensive fighter for the... the... uh..."
Robert McLees: "Earth Space."
Marcus Lehto: "Well, it just didn't fit into our game play, either. We didn't have any kind of purpose for this thing."
Shi Kai Wang: "Unless it's cinematics."
Robert McLees: "Yeah, it'd be a cinematic-only thing, and there wasn't enough time to add it in."
Deleted Material Edit
When developing Halo 2, Bungie faced many delays for various technical reasons. As such many aspects of the story such as vehicles, gameplay/design, and story elements did not make it into the final version of Halo 2. Many of the elements have been confirmed by such things as videos and commentary on the Limited Edition special DVD.