Galesaurus was a prehistoric mammal-like-reptile, incorrectly classified as a dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 by Sir Richard Owen in 1859. Notably, Galesaurus was mentioned in the first issue of Nature

Nature (journal)

Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

 in 1869, where T. H. Huxley expressed confidence that it would eventually be shown to be a dinosaur. However, current opinion is that it was not a dinosaur and belongs to a mammal-like group called Cynodont

Cynodont

Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...

s.