Scientists have dubbed it "the chicken from hell" - and you would not want one loose in your hen coop.
The long-legged, 10ft (3 metre) tall bird-like dinosaur lived alongside Tyrannosaurus rex 66 million years ago.
Anzu wyliei - the first part of its name is derived from a bird-like demon in Mesopotamian mythology - weighed 500 pounds (227kg), measured 11.5ft (3.5 metres) from head to tail, and sported a cockscomb crest on top of its head.
It also had rudimentary feathers, sharp claws and a toothless beak.
Experts believe it was an omnivore, eating both vegetation and small animals.
Dr Matt Lamanna, from the Carnegie Museum of Natural History in Pittsburgh, one of the US experts who described a newly constructed skeleton of the dinosaur, said: "We jokingly call this thing the 'chicken from hell,' and I think that's pretty appropriate."
The bones were found at sites in North and South Dakota in a rock formation known as Hell Creek that has been a rich source of dinosaur fossils, including those of the iconic T. rex and Triceratops.
Although the "new" dinosaur's remains were discovered years ago it took time for scientists to study and fully describe them.
Their findings appear in the online journal Public Library of Science ONE.
Co-author Dr Emma Schachner, from the University of Utah, said: "It was a giant raptor, but with a chicken-like head and presumably feathers.
"The animal stood about 10 feet tall, so it would be scary as well as absurd to encounter."
She added: "I am really excited about this discovery because Anzu is the largest oviraptorosaur found in North America.
"Oviraptorosaurs are a group of dinosaurs that are closely related to birds and often have strange, cassowary-like crests on their heads."
The cassowary is a flightless bird from New Guinea and Australia related to emus and ostriches.
A variety of bones from three of the creatures were pieced together to make up almost a full skeleton.
Two of the specimens showed signs of injury, either as a result of accidents or violent encounters with other dinosaurs.
"One appears to have a broken and healed rib, and the other has evidence of some sort of trauma to a toe," Dr Schachner said.
The Hell Creek formation consists of clay, mudstone and sandstone deposited on ancient flood plains where giant dinosaurs roamed towards the end of their reign, between 66 and 68 million years ago.
Co-author Dr Hans-Dieter Sues, from the Smithsonian National Museum of Natural History in Washington D.C., said: "For almost a hundred years the presence of oviraptosaurs in North America was only known from a few bits of skeleton, and the details of their appearance and biology remained a mystery.
"With the discovery of A. wyliei, we finally have the fossil evidence to show what this species looked like and how it is related to other dinosaurs."