The Thunderbird is a legendary creature associated with the folklore of Native Americans. Although the Thunderbird is generally depicted as a semi-divine entity, it is also believed by many to be a wrathful being, and great care was taken to try and appease the powerful creature, as it was believed to possess an array of supernatural powers by which to cause havoc on those that displeased it.
Much like the Roc, the Thunderbird was believed to be a gigantic bird, and was either seen as an individual or as part of a species. The Thunderbird could cause storms to brew via flapping its powerful wings, and some legends also spoke of the Thunderbird being able to fire lightning from its eyes. The Thunderbird was also known to create thunder, as it flew across the sky, and was, in many ways, a personification of a great storm.
Thunderbird is also classed as a cryptid, though science would undoubtedly be searching for a smaller and much less supernatural bird.