Dimetrodon
Dimetrodon was a carnivore, probably the top predator in its environment. Its main feature is the large sail on its back formed by elongated spines extending upwards from the vertebrae. The spines would be joined by skin. The general opinion of palaeontologists is that it was a temperature-regulating device. The idea is that the animal could warm up in the early morning by placing itself broadside to the Sun, and later could cool off in the shade, or arrange for its body to get less sunlight. At this stage in evolution, no land animals were homoiotherms. Very likely the sail was also used for sexual or territorial signalling. In which case, the skin would be coloured. This, of course, is supposition, but it does make sense of the animal's most extraordinary feature.