Legend says the thunderbird was an enormous bird with a wing-span so large the sweep of its wings clapped like thunder. Also associated with storms, it is believed the thunderbird carried lightning on its back. They are intelligent, powerful, and wrathful.
They seem closest in form to that of an eagle or hawk but they are far stronger and larger in build. Their polychrome plumage gives them a magnificent appearance unrivaled by other birds of this world. Their voices are that of a flute, recalling the sound of a shrill wind or the cry of a raptor.
In the godwars of the 6th Age, it was not the thunderbird which wielded lightning, it was the one riding it. Legend says they may appear alongside balded, powerful men "always ready for war." These were likely those which were trained to ride the thunderbird to war, and whom also carried a club, a weapon from which would shoot fiery projectiles or lightning.
The birds themselves are brave and powerful, but slow-witted unless trained for war. They are the most well-known by the native american peoples of North and Central America and were all at once hunted, feared, and worshiped by these tribes, but exist also in the deserts and flatlands of the world. It is in these landscapes thunderbirds now inhabit, hunting cattle, bison, and other herding animals. If one could be captured it might be capable of training, but they are hard to find and harder to control.