Unused sound files found in Call of Duty 2indicates that the Panther was originally going to appear in that game but was scrapped at the last minute. For example, there's a sound file in which a Russian says "They got a Panther tank!". There is however, no model or texture files for the Panther.
The Panther appear more extensively in Call of Duty: World at War in the Russian campaign - most notably in "Blood and Iron" where they make up most of the German tank force in the level. The Panther was supposed to appear in "Holland", but the level was scrapped - with the model remaining. The tank does not appear in multiplayer mode in favor of the Panzer IV.