Earth dragons appear early in the game. While they and their hybrids don't generate a lot of income, their habitats can store a lot of coin and are thus invaluable to accumulating a lot of wealth early in the game.
"Despite the harshness of their habitat, the earth dragon is the least restless of all known breeds. They are supremely well adapted for their desert homes with thick skin and large bodies to store water. The spikes on their back offer protection from predators, but honestly, what animal in its right mind would think to take on one of these massive beasts?"