Staab's reconstruction of a Phorusrhacoid, more commonly known as a "Terror Bird". And for damn good reason too: Have you ever had an up-close encounter with an ostrich or rhea, or other modern big bird? They're vicious bastards, and will happily peck or kick the tar out of anything that attracts their attention. Now make them carnivorous, 10 feet tall, weighing as much as half a ton, with vicious claws and a meat-cleaver like beak, and you have yourself a Phorusrhacoid, pretty much th baddest-assed thing to come along since the velociraptors. Some theories even site the idea that their short stubby wings (usually interpreted as smaller/less feathery than what is shown here) could have employed a hacking motion not unlike a meat hook. Oh yeah, and since they're fully warm-blooded, and super oxygenated because of the unique arrangement of bird lungs, they probably could have run down your average track star without getting winded.
Phos. gave early mammals a very good run for their money in the early Cenozoic in the race for dominance as land vertebrates, and probably preyed heavily on things up to medium-dog-sized, including early horses, camels, etc. Simply put, had these guys been a little bit more successful (any combination of more fertile, faster, or just plain meaner), we wouldn't be here at all.