Ratigan (also known as Professor Padraic Ratigan) is a rat with mental illness and the main antagonist in The Dark Side of NIMH. He was the mysterious leader of NIMH. Ratigan plotted to rule the world and destroy Thorn Valley. But He was defeated by Woody and Mrs. Brsiby. He died after falling off the top of the stone aged castle following a fierce battle.

Ratigan served as the arch nemisis to BasilOliver and Nicodemus.