Chelonian - 1. Belonging or pertaining to the order Chelonia, comprising the turtles. 2. A turtle.
Hagrid was famous for loving animals, especially large, dangerous ones. If anyone had been asked, they might have said that dragons were his favourite animal, or maybe Thestrals. But they'd be wrong. They weren't.
Nor was the friendly giant squid that lazed around in the waters of the Lake and frolicked with the students on balmy summer days. Another water creature was, though.
Hagrid loved the giant fresh-water turtles that lurked in the murkiest and deepest waters. The ones that had a shell broad enough and strong enough to bear even his weight. He loved riding on their colossal backs.