Flightless, loud af, pigeon-duck-chicken-heron thing living only on remote island far off the coast of Australia - The Kagu

BBC Earth documentaries are the best.

The kagu and South American sunbittern, and possibly the adzebills, seem to form a distinct Gondwanan lineage of birds, either as one order or possibly more. Although the relationships between them and groups previously considered related, such as the mesites and the "core Gruiformes," are not yet resolved. It is notable, however, that the sunbittern and the mesites possess powder down too, whereas the "core Gruiformes" do not. The ancestors of the kagu are believed to have diverged from the sunbittern in the Oligocene, 45 to 17 million years ago, and colonized New Caledonia 60 to 25 million years ago. In the absence of terrestrial predators, it eventually became flightless.

While the kagu is the only living species in the clade Rhynochetidae, a larger species, the lowland kagu (Rhynochetos orarius), has been described from Holocene subfossil remains. The measurements of this species were 15% bigger than Rhynochetos jubatus, with no overlap in measurements except those of the forelimbs. Given that the sites from which R. orarius remains have been recovered are all lowland sites, and that no fossils of R. jubatus have been found in these sites, the scientists that described the fossils have suggested that they represent highland and lowland species respectively. R. orarius is one of many species to have become extinct in New Caledonia after the arrival of humans.

I wanted to check after the video said they weren't related to rails.