The carakiller is a polyborine falconid, descended from the mountain caracara, from the Amazon Grassland, 5 million AD, in The Future is Wild and its reboot.
On a grassy plain, even if the grass is long, there is little cover. Large predatory animals cannot hunt by stealth, so instead they must hunt by speed. Back in the Quaternary, the most famous of the swift-moving grassland predators was the cheetah (the fastest land animal on Earth at that time).
The swiftest hunter of the Amazon Grasslands is a flightless bird called the carakiller.
One of the most common falconiforms of South America during the Quaternary was the caracara. The caracara was a versatile, predatory bird, and easily adapted to life on the plains. Its descendants became large ground-dwelling birds and eventually evolved into a bird that was completely flightless: the carakiller.
The wings of the carakiller no longer have large flying feathers, but they are still muscular and aerodynamic. The bird's flightless wings now act as stabilizers, balancing the carakiller when it runs at full tilt, and helping it to turn corners quickly. In addition, each wing has a long curved claw at the tip.
The carakiller's body is covered in feathers for insulation, shaggy on the back and legs, and fine on the chest. Its neck and wings are bare, presenting smooth surface when the bird is eating (feathers would soon become sticky with the flesh and blood of prey).