Unlike some other unused models, the only evidence of the Panther are pictures. Most others unused enemies were still left in the game's pak file. I don't know much about them except they were very dog like creatures and that they may have been replaced by Hound Eyes.
A close up of a Panther
A panther who caught a scientist
Here you can see Barney throwing a scientist to the Panther
A wireframe of the Panther, notice the cell it's being kept in
In some pics the Panther is red for some reason