It is used by a G.U.N. soldier in an attempt to stop Shadow from harming anyone 50 years ago when Project Shadow was discontinued. In the flashback of 50 years ago, Shadow saw Maria die at the hands of the Heavy Dog's pilot. It wields a particle beam cannon and several weapons on its side which can be broken off and used against it.
This vehicle is fought after Dark mission of The Doom. It is fought on a metal platform with raised platforms; these can be used to avoid the particle beam cannon, however the cannon can also reach this area. The Heavy Dog releases explosives that can be used as stepping stones to attack or disable a cannon that fires the same missile as an RPG. If you attack the Heavy Dog's RPG's enough, then they will fall off Heavy Dog for the player to use.
- The boss theme for Heavy Dog is a remix of the boss theme for the fights against F-6t Big Foot, B-3x Hot Shot, and R-1/A Flying Dog in Sonic Adventure 2.