A giant tardigrade recovered from the USS Glenn whose bond with the mycelial spores briefly made it the navigator of Discovery's experimental spore drive.
Ripper is the giant tardigrade found during the investigation of the USS Glenn in Context Is for Kings. Discovery's boarding party discovered the Glenn's crew dead and a group of Klingons killed by an unknown creature, and Captain Gabriel Lorca quietly had the animal beamed aboard his own ship.
In The Butcher's Knife Cares Not for the Lamb's Cry, Lorca assigns Michael Burnham to study the creature as a potential weapon. Commander Ellen Landry, who names it Ripper, tries to sedate it and cut off a claw, and it kills her. Burnham concludes that Ripper acted in self-defense and is drawn to the spores, and once moved to Engineering it interfaces with the drive and makes the jump to Corvan II possible.
In Choose Your Pain, the repeated jumps are visibly harming Ripper. Burnham and Hugh Culber press Paul Stamets to find another way, and Stamets injects himself with Ripper's DNA so he can navigate the drive himself, after which Burnham sets the creature free.