Terrifying Sample (Dark) comes from Underwater sources & if you have the item in your inventory, increases the chances of the Abyssal Horror jumping out of the Alluring Specimens.

Terrifying Sample (Fear) comes from Space Encounters & other space sources such as Frigate Expeditions & has a strong chance of causing one of the scarier Space Encounters to keep pulling you out of Pulse Drive involuntarily; 'Stalking' you.