Lady Arabella Horsley was a wealthy woman on 21st century Earth. Another woman, called the Unique Lady, became jealous when she found out that Lady Arabella hired the artist Johann Illes to paint her portrait. The Unique Lady felt that if Illes completed this portrait of Lady Arabella, it would make her own portrait by the same artist lose its uniqueness, so she had Illes murdered.
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)