From Lexicanum

Familiars are psychic-symbiotes used by psykers to augment their powers.

Familiars take many forms. Those used by Imperial psykers are commonly cybernetically-augmented or mechanical animals or creatures.

They are mentally linked with their master, which allows its master command it, see and hear through its senses and use its powers. The master is aware of everything the familiar is, and vice versa. If the familiar is injured, psychic feedback will hurt the master as well.

Examples

• Haemoncolytes, symbiotes used within the Adeptus Mechanicus.
• Hamadrya, a unique familiar used by Lufgt Huron.
• Gyrinx, cat-like creatures used by Eldar psykers as familiars.

Sources

• Inquisitor Rulebook, p. 54