|Brontotherium skull - or should that be Brontotherium prousti?|
A genus is a group of closely related species. There are many genera that only contain one species, but most have a number of different species at any given point in time, and even more when you look over the whole of their evolutionary history. It's as if we thought of the living genus Canis as a single entity, ignoring the fact that it includes wolves, coyotes, and jackals, never mind extinct species such as the dire wolf.
There's a reason for this, of course, which is that it's really tricky to tell actual species apart from their fossils. Taking the traditional definition of a species as something that doesn't cross-breed with other species to produce fertile hybrids (and the modern definition is a bit more complicated than that), how on Earth would you know, if you're only looking at a fossil? Instead, the definition more commonly used for fossil animals is that a species is a group of specimens so similar that there's no way to consistently tell them apart, even when you have a complete skeleton. But most fossils aren't complete, or are at least damaged by several million years of being squished under tons of rock, which makes it very difficult to spot the tell-tale differences between the equivalents of coyotes and jackals.