Arabella Horsley

 was a rich lady on 21st century Earth. When the Unique Lady found out that Johann Illes was going to paint a portrait of her, making her portrait lose its uniqueness, she him killed. The Tenth Doctor said that, as there were twenty-three quaggas in museums, anyone with enough money to hire people, such as Lady Horsley, could obtain a quagga coat, distressing the Unique Lady. (PROSE: The Last Dodo)