(More biology stuff than drawing)
(Rainworld spoilers)
I find Slugcat design very interesting, actually. A soft body? Perfect for surviving most falls, flexible, and good for movement. I assume a lot of them have somewhat wet skin, which is understandable for a world of rain. They have thin arms which they could tuck into their bodies while rolling and might not need much muscle in them to throw spears, and they are primarily bipedal so they don’t need their arms for walking. Their back legs are large, allowing them to move quickly and do powerful jumps. But just like humans, their tails are pretty useless, even for balance.
Spearmaster is well designed for long distance travelling, light and seemingly thin. But they probably wouldn’t be suited for staying in one place for long, since they’d likely end up hunting a lot of the creatures into migrating away after a few cycles with how much it takes to fill them up.
Artificer has an interesting design. I’ve seen theories that Artificer has an explosive substance in their saliva, allowing her to convert spears and rocks into explosives. And she can explosively propel herself. Usually only sea creatures can propel themselves in such large and sharp bursts using jets. The fact that explosives don’t harm her could be maybe similar to what woodpeckers use to protect their skulls, which is wrapping their tongue around it. The rest of her body could just be very flexible and just go with the explosion. She can’t go into water, which makes sense if her explosive saliva reacts to too much water.
Hunter is pretty basic. The theory that they attach a spear to a chunk of rot on their back is pretty realistic to be honest, and there’s not much else to it.
Gourmand is just very large and heavy. But a bit of that fat would be muscle with how strong their single spear is.
Survivor and Monk are just… slugcats.
Rivulet is very fast, but there’s not much reason why unless all their power goes into their legs, and has a weaker spear throw. Their ruffles and skin are quite bright, like a lot of fish, and their complex eyes imply they have eyesight similar to ours, so not very good. They likely use their ruffles to navigate underwater and their eyes on land. They would swim using their tail as well as their limbs, but would still probably slow them down a bit on land.
Saint has a lot of fur, which is good for a cold environment. Since they primarily move with their tongue, it makes sense that they wouldn’t be faster than average on their legs. I don’t understand the tongue very much, as frogs and chameleons aren’t adapted to cold temperatures and will either die or freeze, which is what Saint does. They can’t throw spears, so their arms are practically useless for anything but climbing poles.