Type 50

Type 50 was a type of TARDIS used by Time Lords. It was much more advanced than the Type 40s. The First Doctor owned a Type 50 TARDIS before leaving Gallifrey in a stolen Type 40 TARDIS, though never gave a clear reason why in his fifth incarnation why he didn't take the Type 50. He told Nyssa he had to leave "in a hurry." He also tried to apologise to his former TARDIS for hurting it, but it wouldn't listen. (AUDIO: Prisoners of Fate)

Maris heard rumours that some Type 50s could take on human form. This was proven to be correct when the Doctor's first TARDIS appeared to her in the form of a young child and was capable of leaving Gallifrey on its own in a fit of rage. (PROSE: Celestial Intervention - A Gallifreyan Noir)