Lineage System

The Lineage System determines which creature type will re-spawn in a den when killed by the player. This only affects certain dens in game; normal dens only ever spawn a single type of creature.

Explanation
When a creature is killed by the player, there is a 33% (50% for Hunter) chance per cycle of a new creature spawning in the parent den. For lineage dens, when that creature respawn chance is fulfilled, there is an additional chance that a different type of creature, usually another colour of lizard, will instead spawn based on a predefined lineage list. This is the only way to encounter red lizards in game as Survivor and Monk.

Information on Lineage lists for specific dens can be found in Rainworld > World > Regions > [specific region] > World.txt, as well as in this interactive map Made by Henpemaz

Examples
In this image, SU_A40 shows the room code, and : 2 : is the den number. The first type of lizard to spawn in this den will always be green. When killed by the player, there is a 10% chance (0.1 out of 1.0 total) that the next lizard spawning will be pink instead. Every time pink is killed, there is then a 50% chance of changing to blue, and so on. The red lizard here will re-spawn infinitely, since there is a 0% chance of progression. In this image, the NONE value means that nothing will spawn in that den from game start, with a 5% chance per cycle of a pole mimic appearing. After it is replaced with another mimic (40% chance per kill), there is then another 40% chance that a tentacle plant will instead appear.

Curly brackets { } indicate the size of the creature, if applicable, and can also be seen with centipedes.

Death Glitch
In all game versions from 1.5 onwards, injured creatures do not regain health at the end of a cycle and so remain in their den indefinitely. It is possible to despawn entire regions this way. This can be fixed by installing Deathfix, which is also included in the ManyMoreFixes mod.

This glitch is due to be patched out in the Downpour Update.