The Type-one TARDIS was an early prototype created by the elders of ancient Gallifrey. It was poorly designed, dangerous to operate and difficult to start. Some of them had minds of their own, with a history of escaping and roaming the Universe.
The Second Doctor discovered an old, decrepit woprat on a Type-one TARDIS in an Avatar universe. Unfortunately, Jamie McCrimmon killed it before the Doctor could get a closer look. (PROSE: Heart of TARDIS)