Cancer is a disease which can afflict any multicellular life, including plants, birds, fish, dinosaurs and even, on rare occasions, insects. It has been around for a lot longer than humans have.
Cancer, as a disease, probably first arose more than half a billion years ago when multicellular life first appeared on the planet. Many of these early creatures, however, wouldn’t have lived long enough to develop the disease. Cancer would have become more common and creatures grew older.
Aside from the fact that fossils of dinosaurse with cancer have been found, we know that cancer is truely ancient, because we have evolved sophisticated mechanisms for avoiding it. These same mechanisms appear in all multicellular life on this planet, which means that we must have all evolved from the same, common ancestor who had already evolved mechanisms for dealing with it.
To put this in perspective, imagine the whole of recorded human history is about 2.5cm, the length of a piece of chewing gum. In that case, one meter represents the length of time that humans have been in existence. Then, multicellular life and the origin of cancer would be 2000 meters away!! Cancer really is an ancient disease.
As for why cancer is so lethal, look at the answer I gave to this question:
Cancer has probably existed as long as animals have. That’s because cancer is an unfortunate by-product of the way our bodies work: cells have to divide, and each time they divide there’s a risk that the DNA won’t get copied accurately. When the DNA gets changed slightly, we call that a mutation. Most mutations are harmless, but some mutations cause cells to grow and divide too much, and they can become cancer cells.
The reason it’s so lethal is that unless treated, most cancers will keep on growing until they ‘take over’ and the organs of the body can’t work properly any more.