(The largest extinct rodent known to science was Josephoartegasia monesi. It lived three million years ago, was larger than a bull, and weighed over a ton. Like its modern-day relative, the sheep-sized capybara, it lived in South America.
Gigantism is a well known evolutionary response that occurs when a small creature steps into an ecological niche left by a larger species.
Rats the size of sheep! If that isn't a good enough reason to protect the elephant, rhino and other species from extinction, I don't know what is.)