Mermaid (The Southwell Park Mermaid)

The mermaid of Southwell Park was originally one of many humans whose genes were altered by intelligent horseshoe crabs. When the government released a drug-killing bacteria, the altered genes were activated and the mermaid started changing physically, enabling her to survive living in the sea.

The mermaid met the Seventh Doctor, who fed her. She let him take a blood sample. (PROSE: The Southwell Park Mermaid)