Ripper
"Ripper" was the nickname given by Ellen Landry to a large animal discovered aboard the disabled  in 2256, by a  away team. He was dubbed a  due to his resemblance to the Earth micro-animal. According to a captain's log from the Glenn, Ripper was found stowing away in the ship's stores of mycelium spores. ()

Ripper was highly aggressive when first encountered: he had slaughtered a Klingon boarding party and proceeded to chase the Discovery team, killing Kowski before Michael Burnham managed to lead him away from the others. Impressed by the animal's ability to tear through metal and resist phaser fire and Klingon blade weapons, Captain  had him beamed to a containment pen in a Discovery lab. ()

Lorca ordered Burnham and Landry to find a way to weaponize Ripper. Burnham hypothesized that his behavior on the Glenn was defensive in nature and that he was not inherently dangerous, since biological analysis showed him to be herbivorous. But with the Klingons attacking Corvan II, Landry was desperate for results and attempted to sedate Ripper with oneirogenic penthrene mist, so as to remove his claw for study. The anesthetic had no effect, and Ripper fatally mauled Landry after she fired a phaser at him.

Based on Ripper's reaction to the activation of the spore drive, Burnham determined that he had a symbiotic relationship to the spores and an innate knowledge of the mycelial network. This led to the revelation that the Glenn crew had devised a mechanism to link Ripper to their spore drive, solving the problem of navigation over long distances. Using this technology, the Discovery was able to jump to the Corvan II colony and save it from a Klingon attack. However, the process caused visible distress to Ripper. ()

Over the next three weeks, the Discovery made repeated jumps using Ripper. However, Burnham grew concerned that Ripper was becoming overtaxed and asked Doctor Hugh Culber for assistance. Culber came to the same conclusion, and additionally found evidence that Ripper may be sentient. Nevertheless, Saru overruled their concerns as Lorca had been captured by the Klingons, and the Discovery needed to make another jump into Klingon space to mount a rescue attempt. The stress of the jump caused Ripper to enter a state of extreme cryptobiosis.

After Paul Stamets successfully connected himself to the spore drive in Ripper's place, Saru asked Burnham to save Ripper's life. Reasoning that what Ripper ultimately needed was freedom, Burnham released him into space with a supply of mycelium spores. Once outside the Discovery, Ripper revived and jumped himself via the mycelial network to parts unknown. ()

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Appearances

Background information
Originally, the giant tardigrade was intended to be a crew member on the Discovery named "Ephraim" (in honor of , who discovered real-life tardigrades). Ephraim would have been Stamets' superior, have conversations with Burnham, and even wear a specially designed Starfleet uniform, with a sash on which his badge would be pinned. However, the idea proved too expensive. 

Ripper appears in the  Dead Endless, where he is confirmed to be sentient. He prefers being called "Ephraim", another name given to him by Humans. Author Dave Galanter confirmed that this is supposed to be the same tardigrade that appears in . 
Tardigrades