Terror Birds are apex predators. They're tall, flightless animals with long, strong necks and large, hard beaks that can cut through flesh. Their small, atrophied wings bear two small useless claws. They run as fast as horses and hunt with the cunning of wolves.
For a creature with such a dramatic name, Terror Birds are actually pretty benign. In fact, this creature and its name, is an actual prehistoric creature that lived in the Miocene period (23 to 5 million years ago). Their scientific name is Phorusrhacidae and are thought to be the ancestors of Seriemas birds, something a little smaller and less lethal.
If you think these guys are goofy, just put a little though into it. These are extremely fast creatures, up to 3 metres tall (10 feet). The smallest of the species was about 1 metre (a little over 3 feet). They're basically like ostriches. You know how scary ostriches can be? Well, imagine if they actually wanted to eat you.
A nice monster if you want to use it in a prehistoric campaign setting.