Who is Dinosaur from The Diamond Age, or, Young Lady's Illustrated Primer and what is their importance?

Dinosaur is one of Princess Nell's magical friends. These friends correspond to stuffed animals belonging to the real Nell. Dinosaur is a T-Rex that survived the extinction of his species by studying with a mouse called Dojo for over sixty-five million years. He is extremely strong, but he is also extremely heavy, and leaves Nell when his weight overburdens the party.