From what I know, in less educated times (medieval, witch hunt times) cats were witch's familiars, meaning their animal connection to the "evil" that witches did.
Many times the older cast-out woman who lived on the edge of town and had feline companions was thought to be the "witch" and blamed for anything bad that happened. If it was bad enough, she and her cats were killed. There were often times mass-killings of cats because they were thought to be a tie to the devil and evil since they were witch's familiars.
That's the only context that I know of calling cats "familiars". There could be others, though, depending on that person's beliefs.