Krazy-Chibi
I guess the only thing I really came up with was that humans tend to stereotype them as a feral, uncivilized people, (though this is obviously false) though with some rare cases im sure there would be Humanoids that fully give in to their 'inner beast' and become monsters.
These rogue Humanoids could pose as side-villains. \o/
Alright, here are my ideas. I actually ended up having a lot the more I thought about it so this'll be pretty long. Sorry if it's kinda too much. 
● Humanoids would not be too much like weres. They cannot pass their abilities onto the other races, so one has to be born into the race. I could also make specifics on what half-breeds would be like, but it probably won't be necessary until someone wants to make, say, a character who has one Humanoid parent and one Human parent. Or parents of different species.
● Humanoids would have a default form and a Feral form. Default is where they merely look like humans with animal features. Feral is where they look a lot more animal and are bigger and more muscular. But they still have hands and the ability to stand on two legs.
● Even in default form they have great physical strength, and it doubles when they go Feral. They still have their heightened senses in default form, and they also have accelerated healing. I'd say life expectancy for Humanoids would actually be Above Average/Varies due to this.
● Transformations aren't dependent on the moon. It's all pure will. But it does have a high energy cost. Upon returning to default form, one is usually quite tired and/or hungry.
● Humanoids would not have Mystic Artes, but they can willingly transform into their Feral forms upon reaching Overlimit. It's enough that they're total tanks in that form—they can withstand quite a beating from both magic and physical attacks.
● Humanoids wouldn't be capable of magic but still have physical/weapon artes (Tiger Blade, Dual Punishment, Swallow Kick, etc.) at their disposal.
● Intense stress paired with intense anger triggers berserk rages.
● There could be some legend centered around their origins. Maybe something along the lines of...they were animals that prayed to the prime god/goddess (if it's not Luna) for strength and a way to fight better against Humans (and/or other races) because they were changing and destroying too much of their habitats.
In my head, the Merfolk actually turned out kind of differently from the rest of the Humanoids. I'm not sure if these ideas would make them their own separate race or if they can still be grouped with the Humanoids. Most of these ideas came to me because, what if someone wants to make a Merfolk character but most of the RP will take place on land? xd ● Merfolk are very varied in themselves; they can be based on various fish (sharks and manta rays included)
and marine mammals like dolphins, killer whales, seals, and narwhals.
● Unlike the other Humanoids, Merfolk don't exactly have a Feral form. Their default form is their water-dwelling form, in which they have their tail bottom-halves. Their other form is called a Land Form, in which they've morphed their tails into legs and have flipper-like feet.
● Merfolk certainly look more human-like in their Land Form, but would still have the oddest skin colors of all (blue, pink, orange, gray, etc) , as it would match their scale colors or patterns of the animal they're based off of.
● Transforming wouldn't exhaust Merfolk as much as it does the other Humanoids as it's only focused on their bottom halves.
● Also unlike the other Humanoids, Merfolk would have the capacity for water spells and do in fact hold very powerful water magic. Those who hail from arctic waters would also have ice spells on hand. And since they don't have a Feral Form to benefit off of, they could have Mystic Artes.
● Even more, they would not have the same healing qualities the other Humanoids would. They'd heal at the same rate as humans. They still have an Above Average life expectancy, though.
● They become stronger when they're in their element. Out of water, their magic is average in power. But once they're in a body of water, it doubles.
● Their origin legend could still be similar to the other Humanoids', though—perhaps Humans and/or the other races were polluting the waters as well.
