Bird is the word!
The reason I put that praise up there is because Ornithomimids looked almost exactly like ostriches! They were fast, maybe 30 mph even! They were not top predators though, they probably ate rodents and small reptiles. They were also toothless and that explains their diet. They were about 10 to 20 feet long, but surprisingly light for it's size. Probably the most well known Ornthomimid is Galliminus.