From Halopedia, the Halo wiki
Mongoose Mowdown is a Halo 3 multiplayer achievement. It was awarded for splattering an enemy with a Mongoose in a ranked free-for-all playlist. This achievement usually unlocks the SCOUT-class Mjolnir shoulder piece for Spartans, but the shoulders were unlocked without this achievement for some players. The achievement awards the player 5 Gamerscore. The only ranked free-for-all playlist was 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.