From Halopedia, the Halo wiki
Mongoose Mowdown is considered one of the more difficult Halo 3 multi-player Achievements to get, along with Two for One. It is awarded for splattering an enemy with a Mongoose in a ranked free-for-all playlist. This achievement usually unlocks the Scout Armor shoulder piece for Spartans, but the shoulders have been unlocked without this achievement for some players. The achievement awards the player 5 Gamerscore. Currently, the only ranked free-for-all playlist is Lone Wolves.
It is represented by an orange circle with a Spartan driving a Mongoose, three red stars, and the flying body of a Spartan.