Melkur

The Master's TARDIS in the shape of a Melkur and the Fourth Doctor (The Keeper of Traken)

The Melkur was a name given to creatures that were attracted to Traken, meaning literally "a fly trapped in honey". There were several Melkurs attracted to Traken, most decayed without causing any long term harm. 'The Melkur' that seduced Kassia was more virulent than previous Melkurs and was revealed (shortly after the arrival of the Doctor and Adric) to be the Master's TARDIS. The Master utilised the Melkur form to become Keeper and take control of The Source. (DW: The Keeper of Traken)