A Gyrinx resembles a large Terran domestic cat, with very thick and fluffy ginger or orange fur and bright blue eyes. They can grow up to a metre in length. Gyrinx are not truly sentient as such, but they have an astounding ability to empathise with other creatures, forming a mental bond with their owner which is comparable to true friendship. This mental empathy is of great benefit to the creature's owner, whose own speed of thought and action are actually improved while the Gyrinx is in close proximity. What benefit the Gyrinx obtains from the relationship is unclear, but there is plainly some deep-seated need being fulfilled, because an owner-less Gyrinx will actively seek out and adopt another sentient creature. Oddly enough, Gyrinx show a slow metabolic change so that they can come to resemble their owner physically, temperamentally and in habit. If a Gyrinx's owner dies, it will defend the body, and will then either run away or allow itself to be adopted by one of its former owner's friends or associates. It takes a period of several solar weeks to establish a complete relationship with the new owner, but this bonding time might prove to be less if the old and new owners were close friends or relations and the grynix already knew him or her well.
Obviously, Gyrinxes are creatures of great value, and unlikely to be owned by ordinary people. Gyrinx are highly prized as pets by psychic Inquisitors, Rogue Traders and high-level members of the Ecclesiarchy and the like. Evil xenos, such as Dark Eldar, might also own Gyrinx -- the creatures inherit their owner's morality, which in turn can transform them into a being as nasty and vicious as their owners. Eldar Harlequin Shadowseers have also been known to take on a Gyrinx as a companion, and they are popular companions amongst the more psychically potent Craftworld Eldar such as the various types of Seers.
- Rogue Trader: Lure of the Expanse (RPG), pg. 127
- Warhammer 40,000: Rogue Trader (1st Edition), pg. 213