The Iterators are colossal, ancient biomechanical intelligences, each housed within sprawling, intricate metal superstructures, jokingly called a can by said iterators and their creators, the Ancients, that dwarfed the surrounding landscape. These beings were not mere machines but possessed a form of consciousness and the capacity for complex thought and communication, usually through puppet-like robotic avatars often bound to a single chamber by the umbilical chord that keeps the puppet active. Much of an iterator's  consciousness is held within mobile biological components known commonly as neuron flies, which are massed produced in an iterator's neural arrays and are held together with resonant frequencies that can be stretched hundreds of miles, though few are actually that big. Sustaining their immense processing power required vast amounts of water, which they cycled and expelled as torrential rain, fundamentally shaping the world around them. Isolated within their cans, they grappled with existential questions and the legacy of their vanished creators, their interactions often marked by a blend of profound intellect and a weary resignation to their monumental task of solving the great problems the ancients could never solve on their own.

Scavengers, otherwise known as Scavs, are widespread, social creatures found throughout Rain World, typically appearing in groups and communicating through gestures and body language. These resourceful quadrupeds possess long forearms and short legs, adorned with varied antlers, horns, and other unique features, and are capable of wielding weapons like spears and grenades with surprising skill. Scavengers tend to adore and collect shiny objects such as data pearls, which they use as a valuable currency when trading. While not particularly resilient, their numbers and ability to throw projectiles make them formidable, especially in open areas, though they can often be avoided through stealth or by distracting them with larger predators. Notably, Scavengers have a complex relationship with Slugcats, ranging from neutral to hostile depending on reputation, and they engage in trading valuable items, such as data pearls, and guarding toll checkpoints, showcasing a rudimentary yet distinct societal structure similar to feudalism, with a chieftain often controlling literal miles of territory.

The Rot is a tumorous cancer that spreads through various means, that being spreading through and eventually taking over the bodies of those unfortunate to encounter it to Rot cysts eventually growing large enough to break off from their sources and becoming mobile. The Rot can be easily identified by its appearance, often being seen as numerous large, cancerous cysts that each hold a glaring X that marrs each cyst once it grows large enough. The Rot is often portrayed as having a dark blue main color with the X markings often being a brighter blue, but there have been other cases where Rot cysts appear in other colors such as brown and red.

Slugcats are an indigenous species of felinoids. They have two toed feet-paws, three fingered hand paws, lender arms and thick legs, a long body, wide single colored eyes, a hip-wide tail that tapers at the end, and rounded ears. Slugcats average at 3 to 4 feet tall during adulthood. Slugcat are born with fur similar to that of a seal, meaning if it gets even slightly wet it flattens and the slugcat will seem bald. Enough exposure to a specific environment without large amounts of water from a young age can cause a slugcat's fur to become unable to flatten normally, this is why slugcats in snowy area have fluffier fur and slugcats in dry areas have more spiky fur. Slugcats are usually found to be either nomadic or colony living creatures. Slugcats have the ability to swallow small objects and store them inside a secondary stomach to be regurgitated later.