Panther clipper LX, ship that got under the rug, and never came out since. Its supposed biggest cargo capacity cames out also with other problem, too much module space, so it would be too overpowered in certain areas and Fdevs clearly had no idea how they might try balance ship (atleast when it was teased - so at very early stages of game).... and given its size, flying it around surely would make look T-9 like a nible, fast ship, and turing it at supercurz would take over minute to make 90 degree turn and it would be so heavy that jump range would be lower than that of T9... so yeah, I recon only about 1% of cmdrs would actually enjoy flying such brick (probly because mostly of nostalgia from older games), while rest of us probly would only use it for FC refuelling, becasue it would be too slow and bulky to have any chance to compete with other cargo haulers, in terms of speed and efficenty, and again, apart of masochists, almost no one would really enjoy flying it around, due of its near 0 mobility stat, and being slow enough that even CobraIV would be flying backwards faster than this thing would boost in straight line.
Given all that, I am not suprised that ship never came out, and might never will.