Melody

spoiler alert, do not read if you don't know about River Song

On the final day in the human world, Melody said good bye to her love. Her one true love, her Doctor.
He sent her away, into the cyber world, where he hoped she would be safe.
She spent her days caring for those that were also sent there. One day she came across something she had never encountered before.
It was a TARDIS. Though it didn't look like the one she knew. Not like the one that belonged to her Doctor. No, this one was disguised as a wardrobe. A black wood wardrobe.
Melody first noticed it was more than it seemed when she dropped one of her many shoes, and it fell behind her clothes. She reached for it, to no avail. She then pushed aside the clothes, which revealed the TARDIS console. Hardly believing her eye, she stepped into the TARDIS.

So excited was she, that she found her very own TARDIS, that she forgot that a TARDIS can sometimes be unpredictable. The door closed, quietly, but firmly.
Turning around, Melody tried to push the door open again. Her new found TARDIS had another plan in store for her. Instead, it took her to a whole new dimension.
The dragonlands.

Unknowingly she stepped out of the TARDIS once it arrived, without knowing what she would find. Her fragile human lungs could not breath the air of the dragon planet. Her TARDIS, worn from it's long travel, closed it's door before she could get back inside. Using it's chemilisn circut, the TARDIS fades from a wardrobe, and transforms instead into an old, dead looking tree.

Worn out, unable to breath, Melody blacks out.
Her human body dies, but her time lord blood knows what to do. In order to survive this new harsh land, she transforms into a dragon.