“Since the Klingons were the enemy, I had to design a ship that would be instantly recognizable as an enemy ship, especially for a flash cut. There had to be no way it could be mistaken for our guys. It had to look threatening, even vicious. So I modelled it on a manta ray, both shape and color and that’s why it looks as it does in the original series.”
— Matt Jefferies on designing the Klingon ships [source]