Gyrinx
From Lexicanum
The Gyrinx is a cat-like creature used by psykers as a familiar. They were a type of creature in Rogue Trader and were meant to be a pet type creature for psyker characters, including humans and Eldar, which supported its companion by augmenting any psychic abilities the owner possessed. The Gyrinx was also capable of combat, but was relatively weak.
A Gyrinx resembles a large cat, with very thick and fluffy ginger or orange fur and bright blue eyes. They can grow to as much as a metre in length. Gyrinx are not intelligent as such, but they have an astounding ability to empathize 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 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 eventually 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 weeks to establish a complete relationship with the new owner, but might be less if the old and new owners were close friends or relations.
Gyrinx in games are a very useful means of making powerful characters more so. Obviously, these 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-ranking members of the Adeptus Terra, etc. Even evil aliens might own Gyrinx - the creatures inherit their morality from their owner so they can be pretty mean!
The Gyrinx was also mentioned in White Dwarf 105 where it's stated that some Eldar Harlequin Shadowseers will take on a Gyrinx as a companion.
