German heavy SP anti-tank gun. Essentially an 88mm Pak 43 mounted on top of a Panzer III/IV hull (Panzer IV chassis, Panzer III upper hull, IIRC).
Armor was 25mm on the front plate, or 15mm on the casemate/gunshield. Armor was even thinner on the sides and rear, with no top armor. Extremely vulnerable as you can imagine. However, it packs one hell of a whallop, same as the Tiger II. It would be second only to the Jagdtiger, which is extremely unlikely to be added any time soon, if at all.
Now, the reason why we should add this? Its a fairly unique vehicle, that would offer a play style different from that of any other vehicle in the game. Where as the M18 is fast, reaonably armed, and very vulnerable, the Nashorn will be average speed, very heavily armed, and very VERY vulnerable.
Playstyle would likely center on ambush and long-range engagments. If you want to get up close and smack someone in the face for a guaranteed kill, I'm sure it would work, you'd just run the risk of losing perks (not a chance in hell this thing would be free, since the M18 is perked).
Additonally, it gives us a chance to test out turretless TD's in a way the M3 GMC doesn't allow us to do, since it lacks the firepower and optics of a real TD. And think about it this way, even if the turretless TD's turn out to be a flop overall, this will still have a niche as a cheap, effective, but vulnerable heavy-hitter. Sort of like the Firefly, large potential, but more difficult to use properly.