In The Last Continent, this is the supernatural entity who first manifests as a black and white bird perched on the head of the Creator.
A creature like this appears in many belief systems and pantheons, although the jolly name can be misleading. Tricksters have the robust sense of humour that puts a landmine under a seat cushion for a laugh.
Deciding that he either doesn't like this manifestation (the trickster as bird belongs to other cultures, reference Legba the cockerel in Witches Abroad and the magpies in Carpe Jugulum), or isn't keen on guano down his back, the Creator re-draws Trickster as a kangaroo.
In kangaroo form, Trickster then makes contact with Rincewind to tell him he is needed on a quest of the utmost importance, and sticks to him like glue to ensure the task is completed.
Just call him Scrappy.
"Scrappy" evokes the once-popular sideshow attraction of the "boxing kangaroo". It also echoes the long-running Australian children's TV show Skippy the Bush Kangaroo. As with the Librarian's oook, Skippy could communicate volumes by the wrinkling of the nose, with the rubbing of the ear offering useful contextual modulations.
Wiggled just so, kangaroo nasal language can convey a precise meaning, like "Come quick, someone's just fallen down a deep hole", with modifiers for age, gender, and profession. As Rincewind discovered, this one is used a lot. You'd be amazed.
Given the setting, the black and white bird is most likely a magpie.
Ensign Snowy, United States Navy, is one of the Beagle people, the sentient canines from a long way down the Long Earth, who is taken aboard Maggie Kauffman's command (the USS Benjamin Franklin) as representative of his people.