Ambrosius, with Sir Didymus on his back.
Ambrosius is a cowardly Bearded Collie , and is the mount of Sir Didymus in Labyrinth. He seems to resemble Sarah's dog, Merlin (interestingly, in some versions of the Arthurian Legends the wizard Merlin is known as 'Merlin Ambrosius'). He has more sense than his master.
He appears in the 1989 film adaptations when Sarah is trying to cross the bridge to escape the bog of eternal stench.