From Halopedia, the Halo wiki
The Doozy is a snowmobile in service with the United Nations Space Command. It is used for rapid movement over snowy terrain, particularly for medical transport, messaging, diversionary tactics, and evacuation. A rear-facing "nausea seat" can accommodate a passenger, much like the rear seat of the M274 Mongoose. However, the design is often seen as a hindrance.
The Doozy is inspired by a boat of the same name that was implemented in early builds of Halo: Combat Evolved though ultimately cut. The watercraft would have featured a rear-facing seat, allowing a player or NPC to engage pursuing enemies. The Doozy, along with several variants of the M12 Warthog, was canonized in the Halo Encyclopedia (2009 edition), albeit as a snowmobile rather than a boat.