"Terror birds" is the popular name for Phorusrhacidae, a family of large, mostly flightless predatory birds. As a family, they existed for an unusually long stretch of time — from roughly 60 million years ago, not long after the dinosaurs' extinction, until as recently as about 1 to 2 million years ago, with some evidence suggesting isolated populations may have survived even later.
Built to hunt on foot
Terror birds couldn't fly, but they didn't need to — their long, powerful legs made them fast runners, and some species are thought to have been capable of real bursts of speed while chasing prey. Their most distinctive feature was an oversized, hooked beak, used to strike, grip, and likely kill prey with a hatchet-like blow rather than a crushing bite.
Sizes varied a great deal across the family. The largest known species stood roughly 2 to 3 meters (6.5–10 ft) tall, while many other terror bird species were considerably smaller.
South America's top predator, for a while
For most of their existence, terror birds thrived in South America partly because the continent was geologically isolated, cut off from the large predatory mammals that dominated the rest of the world at the time. That isolation let birds, rather than mammals, become South America's dominant land predators for tens of millions of years.
The end of their isolation
Around 2 to 3 million years ago, the Isthmus of Panama fully formed, connecting North and South America and allowing large mammalian predators to migrate south for the first time in tens of millions of years. That event, part of what's called the Great American Interchange, roughly coincides with the terror birds' decline, though the exact relationship between the two is still debated.