Creature Spawns

Open the world_XX.txt file located inside of the desired region's folder which you want to edit the creature spawns of. Region folders are located in "Rain World/World/Regions". Towards the bottom of the file you will find the creature spawn definitions.

Here is an example creature spawn line: SB_J03 : 3-Black, 7-Black, 4-TentaclePlant, 5-Small Centipede-3, 6-Centipede-2 The text before the semi-colon is the name of the map that the spawn information is being defined for. After the semi-colon are all the creature den definitions for that map. Each creature den definition is formatted as "X-Creature-Y". X is the numerical identifier of the pipe that will be the home location for that creature. Creature is the name of the creature you want to spawn from that pipe. Y is optional, but it is the number of creatures to spawn. For example, "5-Small Centipede-3" will spawn three small centipedes from pipe #5.

A few creatures have additional parameters that can be defined. For example, "9-Centipede-{5}-2", will spawn two centipedes, each of which are size 5 centipedes, from pipe #9.

A list of available creature names
NOTE* Garbage Worm requires a special pipe connected to it when making the room in the editor.

Here's a doc that shows alternative cases and spellings you can use for each creature.

The Offscreen Den
The offscreen den is a "map" used by creatures which "warp" between maps around the region. This includes Scavengers, Vultures, Leviathans, and Miros Birds. From the offscreen den they can travel to any available spawn destination in the region. Then when these creatures go off-screen, or enter an exit pipe, they return to the offscreen den. Here is an example spawn line for the offscreen den: OFFSCREEN : 0-Scavenger-16, 0-Vulture-5, 0-Leviathan-2

Checking Pipe Information
While in-game, and with dev mode activated, you can check various information about the pipes in the room that will be important for properly setting up creature spawns. Pressing the 'M' key will display a number next to each spawn pipe/location, representing the numerical ID of that spawn. Pressing the 'H' key to open the main dev mode interface, and then navigating to the Map view, will let you see what kinds of spawn locations are available for each map. These are represented by the colored squares under each map. You cannot spawn a creature from the wrong type of pipe. For example if you try to spawn a Scavenger from a pink pipe, or try to spawn a Lizard from a brown pipe, etc... those creatures will not spawn.
 * Gray Squares are for pipes that travel between adjacent maps. These should not be used for creature spawns.
 * Light Blue Squares are spawn locations in the sky for Vultures.
 * Dark Blue Squares are underwater spawn locations for Leviathans.
 * Dark Gray Squares are travel locations for Scavengers.
 * Brown Squares are spawn locations for Garbage Worms
 * Bright Green Squares are spawn locations for Batflies
 * Pale Green Squares are side entrances that can be used by Miros Birds
 * Magenta Squares are the standard creature spawn locations for all other types of creatures.

Refreshing Region Spawns
The current spawned creatures in a region is linked to your save file. So if you make any changes to the spawn information, and then load your existing save file, you will not see your changes. Or at the very least, it will take many cycles before those changes might appear. To see your changes, you need to either start a new game, or use a save file from before the edited region was visited for the first time.