Complex sentence: Clampett designed Tharks, the Green Martians of Barsoom, which he attempted to give a believable appearance, and produced footage of them riding eight-legged thoats at a gallop, which showed all eight legs in coordinated motion.
Split into simpler sentences: Clampett designed Tharks, the Green Martians of Barsoom, which were realistic in appearance. He produced footage of them riding eight-legged thoats at a gallop, which showed all eight legs in coordinated motion.