Size and Weight:
Sometimes it seems that every contemporary animal had a plus-sized megafauna ancestor during the Pleistocene epoch. A good example is the Auroch, which was pretty much identical to modern oxen with the exception of its size: this "dino-cow" weighed about a ton, and one imagines that the males of the species were significantly more aggressive than modern bulls. (Technically, the Auroch is known as Bos primigenius, placing it under the same genus umbrella as modern cattle, to which it's directly ancestral.)
The Auroch is one of the few prehistoric animals to be commemorated in ancient cave paintings, including a famous drawing from Lascaux in France. As you might expect, this mighty beast figured on the dinner menu of early humans, who played a large part in driving the Auroch into extinction (when they weren't domesticating it, thus creating the line that led to modern cows). However, small, dwindling populations of Aurochs survived well into modern times, the last known individual dying in 1627.
Back in the 1920's, a pair of German zoo directors hatched a scheme to resurrect the Auroch via the selective breeding of modern cattle (which share virtually the same genetic material, albeit with some important traits suppressed). The result was a breed of oversized oxen known as Heck cattle, which, if not technically Aurochs, at least provide a clue to what these ancient beasts must have looked like. Still, hopes for the resurrection of the Auroch persist, via a proposed process called de-extinction.