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 it was never added to the game. Little is known of the vehicle, except for a description and a concept art found on the bonus DVD of Halo 2 Collector's Edition.
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."
The Strike Fighter bears a strong resemblance to the YSS-1000 Sabre in Halo: Reach and the jumpships of the later Destiny. It therefore seems likely that Bungie reconsidered the idea of incorporating a usable space fighter in the game.