Plankton

Plankton is a small white rat owned by Jos & Melba Scatterhorn. Plankton liked biscuits a lot. After Plankton's death August Catcher stuffed him and preserved him with the Elixir of Life.

The Museum's Secret
Tom first met Plankton as Melba's annoying pet rat. Don Gervase then stole him and killed him to find the real Elixir; after being revived plankton escaped and somewhow ended up back at the Scatterhorn Museum.

The Hidden World
Plankton appeared as a cameo when Tom visited the museum again.

The Forgotten Echo
In the far future, as Plankton's head was stuffed with a novel about the best escape techniques which helped Tom escape from the Asylum.