Didelphodon was a Late Cretaceous North American marsupial mammal. It was actually one of the largest mammals alive at the time, as it grew to the size of a cat. Nearly all the other mammals were shrew-sized.
In Walking with Dinosaurs the creatures are portrayed as egg-eaters, raiding the nest of a Tyrannosaurus. One was eaten on their first try, but the second time they were lucky, and fought over the eggs. They are also seen eating a baby Torosaurus, that was killed earlier by a pair of Dromeosaurus.
They are depicted (both physically and behaviorally) much like raccoons and badgers.