Reduced stats for subhuman ability

I haven’t seen any mechanic to reflect creatures with significantly below average strength, agility, intelligence, etc.

Could you maybe offering Disadvantage 1-3 as an option for really stupid or weak creatures (compared to 0 being ‘average human’)?

I’m thinking that an average cat or dog sized creature should be WAY weaker than an average human unless there are magical reasons for them to be otherwise.

0 isn’t an average human, zero is completely untrained and relying entirely on luck. An average human has a few attributes in the 1-3 range.

The attributes aren’t a linear scale, unlike some systems; having a Might of 9 doesn’t mean you can lift a car even if someone else with a Might of 7 can. You attribute score is a measure of how effective you are at using that particular attribute.

Another thing: if you’re making creatures that are that much weaker than the players, why would they even need attributes to begin with? A PC is unlikely to be getting into a fight with a cat, and if they do then it’s obvious who will win so there’s no need to roll any dice. Remember that Every Roll Matters so if a situation comes up that is as one-sided as you suggest you shouldn’t make the player roll at all.