The Tenth Doctor once used the cloudrunner to cheer up Martha Jones. When she became angry at humanity for having killed off the quagga, the Doctor cited the cloudrunner as an example of the good in her species. He noted the wonder in the very naming of the species, calling it "brilliant" that some human would invent the name "cloudrunner" for "fluffy rodents sitting about high up in the mountain treetops" instead of going for the more mundane "tree rats" or "mountain mice".
According to The I-Spyder Book of Earth Creatures, it was from Ilin Island in the Philippines and was essentially a big rat with prominently long, furry tail. It was last seen on Earth in 1953 and died out because its habitat was destroyed. (PROSE: The Last Dodo)