Large flying birds called teratorns
Giant bats such as the Ahool and the Olitiau
Flying reptiles of the pterosaur grouping (like pterodactyls)
First documented by Dr. Ernest Bartels, the Ahool (named for its vocalization), was seen in 1925 in the rainforests of Java, Indonesia. Bartels described the Ahool as having a wingspan of 10 feet, a body covered in gray fur, and leather like wings that included claws on each forearm. Although the Ahool is considered a thunderbird it's appearance may be more inline with that of a large bat, the wingspan is almost double that of the largest known bat, the flying fox. A similar creature called Olitiau was first reported in 1932 seen in Cameroon, the Congo, Zimbabwe, and Zambia. Ivan t. Sanderson and his partner, Gerald Russell, claimed an Olitiau came after them. This report from Sanderson and Russell crosses over with sightings of the Kongamato. The Kongamato, whose name means "breaker of boats" has been seen in Zambia, Angola, and the Congo. Frank Melland wrote about this thunderbird in his book In Witchbound Africa. Melland describes this version of the cryptid as having a wingspan of four to seven feet, leather like wings, and either dark red or black in color. Local tribespeople were presented with images of pterosaurs and identified the thunderbird as one. Melland's experiences 33 years later were confirmed when engineer JPF Brown also saw this creature. A year after Brown's sighting a patient in the Fort Rosebery area (where Brown spotted the Kongamato), came in with a large wound to his chest. the man claimed a large bird-like creature was responsible, he drew what attacked him and there was a strong resemblance to a pterosaur, unfortunately the man's drawing has since been lost or destroyed. Another pterosaur type thunderbird is the Ropen of Papua New Guinea.
Yaqui, Sioux, Illini, Hoh, Quileute, and the Indians of Mexico and South America all have stories that describe a pterosaur-like thunderbird. The Illini thunderbird was mentioned earlier this week in the "There be Dragons!" post, the Piasa Bird. Considering the majority of thunderbird sightings describe the flying reptile its easy to see how Indians believed the Piasa Bird is a dragon. John Russell, a writer from Illinois, in 1849 visited caves where the Piasa Bird was said to live. His exploration of the caves revealed, "one mass of human bones." His discovery of the human remains support the idea that the Piasa Bird was a maneater. Even the Quetzalcoatl of Latin America can looked at as a pterosaur; some pterosaurs did have down feathers on their bodies or leathery wings. According to Sioux legend, a hunting party witnessed a giant bird falling to earth after hearing a loud clap of thunder. When they crossed the land to where they saw the bird land, nothing but its bones were left after its fall. It is said that this giant bird had claws on its wing and feet, its beak was long and sharp (some say it had razor sharp teeth as well), and a long bony crest on its head.
The most recent reported sightings of a thunderbird are from 2001 in several cities of Pennsylvania. It is believed that the creature these witness might be seeing are related large birds such as the Wandering Albatross, Andean Condor, or Californian Condor.