AT-AT 300 was the designation, or callsign, for an All Terrain Tactical Enforcer belonging to the Grand Army of the Republic. This vehicle was piloted by multiple clone troopers from Execute Battalion when they infiltrated Cad Bane's Munificent-class star frigate during the Battle of Devaron.
According to the original episode summary found on StarWars.com, the walker was referred to as ATAT300 in the episode, but the correct designation for that particular walker should have been "AT-TE".