A panic creature was a small tentacled, green creature that fed off of fear. The more fear that was generated, the more terrifying the panic creature could grow. The reverse applied and in a calm peaceful state the panic creature was harmless.
On the planet Arabia a panic creature was lurking for years in the Emperor's panic room until the arrival of the TARDIS pushed the Emperor to new levels of fear. But the growth was short lived when the Emperor was quickly calmed down with a chill-out zone. (COMIC: The Panic Room)