A cobblemouse was an animal native to Gallifrey. When they matured, cobblemice grew wings.
The First Doctor once caught a cobblemouse in the mountain above the House of Lungbarrow. He let the cobblemouse loose in the House after his father and Mr Saldaamir ignored him. The cobblemouse interrupted the plans that they were making. (PROSE: Unnatural History)
When the First Doctor returned to Gallifrey before his first regeneration, his cry of frustration startled a cobblemouse and it took flight. (PROSE: The Three Paths)
Marnal had once seen a dead cobblemouse infested with maggots. The sight of a planet inhabited by Vore reminded him of this. (PROSE: The Gallifrey Chronicles)