Very good. It has the speed and attack of the raptor. It also has a glide thing. So if you jump off a mountain or you start to fall, you can hold down space and when you touch the ground their is no damage. Very useful mount for taming, gathering meat and hide, and exploring.
Everyone seems to limit this to just being a "scout". If you get a high level (170+) the terror bird can easily be one of the most deadly mounts out there. It's speed is unmatched and its ability to glide from any height, if it has the stamina that is, makes it amazing. If you pump HP, speed, stamina, and later on movement speed, you will have an absolute killer. (WARNING: Even high level terror birds will have low weight, only bring your essentials to maximize on speed!)
Perfect mount to explore on the ground. Everyone uses a ptera to get around the map, but on the ground it's a whole different experience. Also this mount is my go to for cave exploring. 300+damage and 180+movement speed is a must.