An arctic hare kept in a cage in the Menagerie on the Arcana level of the Mysterium, first observed by Tharivol and Stazi during their ill-fated foray with Elder Lythiin and the Stewards.
Bash then discovered the animal curled up with Nheb‘s corpse outside of the Library of Blasphemous Texts in Chapter 177. He scooped the creature up and tucked it into his shirt for protection.
The hare succumbed to the nauseating stench of the Spawn of Yog-Sothoth in Chapter 178, but received some dedicated healing from Bash in Chapter 179 as the party was moving from the Arcana to the Cipher Obscura level.
First Reference: Act IV Interlude Part 5
Other Notable References: Chapter 177, Chapter 178, Chapter 179