The crew of the Enterprise first encountered Tribbles in the aptly-titled The Trouble With Tribbles, the 44th overall episode of the original Star Trek series.
Happily, the species was re-introduced after a time-travel mission back to the era of the original Trek to prevent Captain Kirk from being assassinated on the space station from The Trouble With Tribbles episode.
Star Trek: Discovery executive producer Alex Kurtzman has confirmed that the new show will include Tribbles, the fluffy little fast-multiplying creatures first seen wreaking adorable havoc on a space station in one of the most memorable episodes of the original Star Trek series.
A sequel to The Trouble With Tribbles episode called More Tribbles, More Trouble was actually planned for Season 3 of Star Trek: TOS but was scrapped, with the story later being used for an episode of Star Trek: The Animated Series.