Dungeons and Diminutive Ponies.
Guess it depends on if this is a game run by a person for fluffies or if it’s a game they expect fluffies to run for other fluffies…
some of the stuff might need to be simplified since math and figuring out ways to tweak might be a bit hard for most fluffies so it might be pretty vanilla. But that just means you wouldn’t necessarily have to tune up encounters for munchkins, unless you have munchkin horses.
Sketti / nummy scented lures that cause a box to drop or something similiar is simple, survivable and potentially solvable with teamwork.
Twine/vine snares might be fixable if the cord is in bitting/curved horn range or allies.
The sketti mimic might be interesting, especially if it becomes a matter of who can eat whom faster. xD - its insides are filled with hot Marinara which will skald the fluffy over time if they cant get out etc.
A fluffy arcane trickster rogue with mage hand would be pretty powerful compared to a normal fluffy trying to disable devices/open locks etc ; since otherwise most of these situations they have to use creative thinking to get around / time things. A pegasus / alicorn would be fitting for those that can use mage hand if only for MLP horn glow shenanigans.
In early levels, much of their potential xp would probably be ‘imagined threats’ like very tiny bugs that cant hurt you but they might be able to bat away or smoosh. Or play fighting / sparring with other fluffies. Overcoming fear based encounters of the mind and becoming braver so they can face off with actual dangers instead of merely imagined dangers…
Humans should be mythical and potentially akin to dragons in CR and it would take some very creative team work / situations for them to take one down. While some might be good (aka hug boxers equal metallic) others can slaughter entire settlements without much effort (aka abusers).
Remember ‘Surving’ encounters still earns some xp even if they dont kill the thing. So there’s creative ways for them to earn xp, especially through social means. ‘Defeating’ doesnt always mean ‘killing’ either so they can best some creatures and force them to retreat etc.