ATAT300 was the callsign of an All Terrain Tactical Enforcer within the Grand Army of the Republic that was used by several clone troopers of Execute Battalion to board Cad Bane's Munificent-class star frigate during the Battle of Devaron.
The old episode guide on StarWars.com states that although it was designated ATAT300 in the episode, the proper designation for the walker would be "AT-TE."