Adaptation Displacement: Yes, Felidae was based on a book. Also, there are actually seven books (the first one being the one made into a film). However, only the first two (and more recently, the fifth) have been translated into English.
All Animation Is Disney: Averted. This is an animated film that looks like it was made by Walt Disney or Don Bluth, but it shows cats getting killed and having sex.
Audience-Alienating Premise: Felidae is a Film Noir with blood, gore, murder, sex, and a cult. Starring cats. Its look made many people think it was a Disney-type movie, but the actual content is not kid-friendly, scaring away both kid and adult audiences. The film has since gained a cult status online, however.
Big-Lipped Alligator Moment: The sex scene, which comes literally out of nowhere, with a character we'd never seen before, and thoroughly distracts the protagonist from his hunt for the killer. Her breed - which he does not recognize - does get him thinking about the case eventually, though, and ultimately points him toward the real killer.
More evident in the books, but the friendship between Francis and Pascal might come off as this to some.
Kong and Francis can give off this vibe, at least in the English movie dub. Kong even calls Francis "cutie pie." He also disparages Francis and implies he's Bluebeard's gay lover.
This is probably perfectly intentional. Cats aren't picky in mating season and homosexual behavior among them is a pretty well documented phenomenon.
Francis's narration directly denies this trope (and indeed, he is even shown to be homophobic) in Salve Roma!, which has a confirmed homosexual character, Antonio. Still, despite his discomfort with this particular trait, Francis is still able to be friends with Antonio. The whole scene is simultaneously hilarious and heartwarming.
Moral Event Horizon: If Claudandus hadn't crossed the line with Felicity's murder, then he did so with Solitaire, who was pregnant at the time. It all becomes moot as shortly after, we find out that the murders have been going on for years and many victims were young or pregnant, thus crossing the line much earlier than we had expected.
Popular with Furries: The film tends to creep out non-furries and non-xenofiction fans as they're not used to cute cats being so violent. Furries and fans of other cat works, like Warrior Cats, however aren't phased as easily. Felidae is thus a pretty popular cat animated film.
The most popular pairing in the fandom seems to be Francis/Felicity even though Felicity is decapitated only minutes after she is introduced in the film and Francis's mate ends up being Nhozemptekh AKA "The cat who had sex with him in the movie." That's because of whatFelicityis. Unfortunately, she's also a Sacrificial Lamb who is Too Good for This Sinful Earth.
Funny enough, Nhozemptekh is barely even mentioned in the second book. She is mentioned, albeit extremely briefly, towards the very end of the book. Perhaps unsurprisingly, Francis comments on all the sex he's had with her.
Francis's ship for the second book is Alcina, a wildcat. She is murdered partway through the book, leaving Francis rather devastated.
Signature Scene: Felidae is largely known only for the very explicit and realistic sex scene between two minimally anthropomorphiccartoon cats. To a lesser extent, it's known for its most gory scenes, such as the one with the disemboweled pregnant cat.
The lynx from the second book. He was brought to Germany from Canada with a group of other lynxes to repopulate the wild. However, since the Canadian lynx's primary food source, the snowshoe hare, was no longer available to them, some members of the group starved to death. A few others were shot by farmers after they attacked livestock in desperation, and the remaining lynxes eventually scattered. He's now alone, and will probably remain so for the rest of his life.
Francis. Moreso in the books, wherein the things he sees and goes through have a much more obvious effect on him - more than once he breaks down crying. (Can you really blame him?)
Isaiah. He's an old cat on the verge of losing his sanity, believing himself to be Guardian of the Dead, and is utterly crushed when he feels that The Prophet (who is actually Pascal/Claudandus) has abandoned him. However, at the end of the novel, he is finally persuaded by Francis and Bluebeard to step into the light and is adopted by a bartender.