Area 51 on Xbox — No Such Game Exists

There is no game titled "Area 51" released for Xbox (original).

Games Named "Area 51"

• Area 51 (1995) — Light-gun arcade (Atari), ported to PS1, PC, Saturn. No Xbox.
• Area 51 (2005) — FPS (Midway), released on PS2, Xbox, PC. Multiplayer: competitive only (DM/CTF), no co-op campaign.
• Area 51 (2011) — Facebook/Flash game (social). Defunct.

Xbox Co-op Shooters with Similar Vibe

• Halo: Combat Evolved / 2 — Campaign co-op (split-screen + System Link)
• Gears of War — Cover shooter, 2-player campaign co-op
• Borderlands (1/2) — Looter-shooter, 4-player co-op
• Left 4 Dead — 4-player survival co-op (Valve)

The 2005 Area 51 on Xbox has multiplayer but it's versus-only. For co-op alien shooting, the above are superior.