Nothing much I want since you’ve removed humans from the equation, but I can suggest ideas in general. Though torment of real owners with kidnapped pet corpses and random surprise Fluffies airdropped over unsuspecting towns which lures even more are better ideas than most things you could have happen to a Fluffy.
Granting them human intelligence on purpose, then doing heinous things anyway.
Unlocking the restrictions in their programming, attaching a Fluffy brain to a human one in a jar which is connected by tubes and wires back into the Fluffy’s skull, magic, and so on. However you want to explain it.
Its not Anthro, but it is removing the separation between the anthropomorphic horse toys and another human. No handwaves like not having souls, running off literal computer if/then/else programming, lacking the ability to feel more than one emotion at once, or any other reasoning can apply. The torturer seems far more vile and the Fluffy’s pain is intensified with complete understanding.
Basically everything else has been done. Its a far more elaborate setup for anything to hit beyond standard creative Abuse, and even then most things would be for the human or some kind of highly intelligent Fluffy.
Being more general you have tarantula hawk wasp and bullet ant venom, the most painful things we know of that are not lethal. Barring the idea Fluffies can spontaneously sad themselves to death from fragility anyway. While the Elfen Lied “you hit me with the worst pain, now there is nowhere for you to go so I can withstand all else” logic can be used in some cases and after exposure then all other sources of pain for the rest of that Fluffy’s life may be highly mild, but those literally cannot be resisted because their chemical nature forces a response, they interject you with unignorable pain. Just hook a Fluffy up to a steady drip. For a visual you can look up people exposing themselves to it on youtube.
You have some of the horrid myths of slavemasters in the American south telling slaves they’d be slaves in the afterlife too but without rest. Except making it real.
A self-perpetuating mill powered only by sunlight and otherwise fully automated where Fluffies are birthed, raised, and brutally mutilated then fed to other Fluffies in the start of the cycle. Like Carpdime’s Fluffy hunting ground comic, only its a tall factory fenced off on some rich dude’s estate where its designed to just run without aid so he always knows that there is always ten or so Fluffies dying terribly back home and that it will run for at least a century after his death before the first parts start breaking down, but even then it’ll still probably be causing suffering for many years before fully breaking down. Maybe fill them with hope, each age they are brought up a level and they can’t wait for sunlight but conditions get far worse each level until they see sunlight for the first time but are skinless and crippled with muscle ripped off their flesh by needles fiber by fiber, final execution coming after a prolonged siren in their ears so they die at maximum terror.
Maybe the worst would be introducing a mutagen of some kind that would mutate them all into more xenophobic brown cross-eyed Alicorns, maybe saying it spreads through air and manufacturing it in a common product nobody would suspect so it happens almost everywhere, maybe in common cleaning agents. Fluffies would all hate each other, hate themselves, kill each other, breed only through rape with a high chance of females resisting their attacker and sparing no offspring ever. Public loses interest as all novelty of form dies off and Fluffies go 99% extinct.