Pilot fish were little scavenger fish that swam alongside huge sharks in Earth's seas. The Tenth Doctor used them as an analogy to describe the Robot Santas and the robotic Christmas trees, scavengers working near a larger predator — in this case, the Sycorax. (TV: The Christmas Invasion)
A pilot fish, as shown in a video found on the internet by Mickey Smith. (TV: The Christmas Invasion)