Dinosaurs
The dinosaurs came about some 230 million years ago and over the following 160 million years developed into a variety of distinct creatures. Some did quite well, others not so. But what really did them in, all in one fell swoop, was an asteroid falling from the sky.
That piece of extraterrestrial rock, estimated to be only a few miles in diameter but travelling at a speed of 30+ miles/SECOND caused tremendous mayhem here on this planet. The impact, known by geologists as the "KT-boundary" event occurred some 66 million years ago and resulted in the Cretaceous–Paleogene extinction event, a mass extinction which destroyed a majority of the world's Mesozoic species, including all dinosaurs, except some early bird and reptile species.
Tyrannosaurus rex and all the other large dinosaurs disappeared, more or less instantly, at least in the geologic time scale.
The few species that survived that cataclysmic impact were small creatures, living mostly in burrows and caves, barely clinging to life, even under favorable conditions. Somehow they survived and over the next 60 million years grew into a large group of mammals. Hominids didn't show up until the end of that period.