Supposedly a photo was taken but it has been lost. The story goes that they are supposed to have dragged the creature back to town and it was measured with a wingspan the size of a barn.

It is more similar to a prehistoric pterodactyl than the traditional thunderbird. It had smooth skin, featherless wings, and a face that resembled an alligator. There is a story from April 1890 where two cowboys in Arizona killed a giant birdlike creature that had a huge wingspan. The Beast of Gevaudan: Monstrous Wolf or Something Else?.

This does not seem outside the realms of possibility, and there is a surprising amount of evidence for a real bird. So, a large and dangerous bird, not necessarily hostile unless provoked, which inhabits the highest peaks and which is only driven low enough to be seen when the weather in fierce. Other stories with more grounded descriptions may help us to get closer to what the real bird might look like. These seem to be exaggerated tales to emphasize the power of the creature, and (unless the thunderbird ranges across the entire North American continent) many of these stories may indeed be mythical, or at least hearsay. Nor be so large that it could pick up a whale in its talons. It would not be able to shoot lightning from its beak from example. It is important to separate what a thunderbird might actually be, from some of the more fanciful legends that are associated with it. The Native Americans held the thunderbird in high regard, believing it would protect them from evil spirits. Using its large wings, the thunderbird could create thunderstorms, bringing rain and storms that were both good and bad. It is an enormous soaring bird, similar to an eagle or condor but much larger and stronger. Almost the only consistent thread between the stories is that the bird has supernatural powers. The appearance of the thunderbird is difficult to pin down, and it has a variety of descriptions that range from tribe to tribe.

Thunderbird mythical birds