Giant water hen
François Leguat (c. 1638/1735) was a French explorer and naturalist who became a refugee and lived on the then-uninhabited Mauritian island of Rodrigues in the Indian Ocean with a handful of other people. Rodrigues was notable for describing various now-extinct birds and reptiles native to the Mascarene Islands such as the saddle-backed Rodrigues giant tortoise and Newton's parakeet. One of the strangest birds he described was le géant, or the giant water hen (Leguatia gigantea).
In the 1690s, Leguat claimed to have encountered a massive white rail-like bird on the island of Mauritius. The bird had a long neck and legs, with a goose-sized body, and a pointed, duck-like bill. It had white plumage, except for the undersides of its wings, which was red. Though its anatomy and geography suggests that Leguatia gigantea was a rail, or at least in the Rallidae family, he described it as being nearly six feet in height, making it three times the size of any other rail species, let alone any non-ratite taxa (i.e., emus, ostriches, rheas etc).
If Leguatia ever existed, it almost certainly would have died out by at least the 19th century, as practically every major flightless bird in the Mascarene Islands went extinct due to overhunting and introduced predators. Since leguatia had such a credible witness and a likely explanation for the lack of other sightings, some ornithologists argue it likely existed. On the other hand, others argue it didn't exist due to the lack of fossil remains and remarkable anatomy.
Some have argued Le Guat misidentified a flamingo, as a population of them existed on the island during his sighting. Flamingos gain their distinct pink colour from the beta-carotene in the crustaceans they eat. If flamingos don't have any crustaceans in their diet, they turn whitish, with the underside of their wings being the last to lose their pink colour, similar to that of the bird Leguat spotted. Due to a lack of archaeological research and acidic soil breaking down animal remains, leguatia's precise origin remains unknown.