The Thraskins, also known as the Cytherians, were one of the original inhabitants of Venus.
The Cytherians were the first civilisation on Venus, living in its early history. They accidentally used up all their fossil fuels and produced too much carbon dioxide, causing a runaway greenhouse effect that caused the collapse of their ecosystem. They were forced to escape the planet or preserve themselves for when the planet was inhabitable again. The Cytherians who stayed behind took samples of all the animals and plants of Venus to be released with the Cytherians. They also produced Vepaja, who would guard and teach the Cytherians when they were awoken.
Millions of year later, human colonists terraformed Venus, making it suitable for the Cytherians. Empress Vulpina happened to encounter the cave where Vepaja was kept. She claimed she would help him, but instead she released the stored species and took the Cytherians, calling them Thraskins. The Thraskins were used as pets or for menial labour. This continued for 6 years, until the Sixth Doctor, Jago and Litefoot came to Venus. The Doctor was able to figure out what had happened and went to the cave where the Thraskins were preserved. Empress Vulpina tried to kill them to protect her secret, but Vepaja decided it was an act of war and destroyed the Venusian city of Amtor. He tried to kill the survivors with a herd of Shanghorn, but Jago and Litefoot were able to put them to sleep with music. The Doctor was able to negotiate peace with Vepaja, allowing the Thraskin and Venusians to live in peace. (AUDIO: Voyage to Venus)