From publisher blurb:
Spontaneous generation, the idea that life can arise from non-living matter, is one of several obsolete biological concepts that can be used in fantasy games to provide a storyteller with more options for monster and setting design. This concept can include living creatures that, by modern definition, are not biological, such as those made of glass or metal.
There are at least three forms of spontaneous generation and these are all covered in this mini-sourcebook. Happenstance due to magical laws is the first and the most similar to the historical definition of spontaneous generation. Alchemy is the use of natural laws to create creatures through spontaneous generation by mortals. Divine intervention involves creation of monsters when circumstances fit the requirements dictated by a god.
“Spontaneous Generation Monsters” includes guidelines on how to create, use, and incorporate into games the title sorts of creatures; descriptions of the three distinct sorts of spontaneous generation; and 17 sample monsters, two of which include variants.
Material in this publication was written specifically for the "Basic" version of the OGL/d20 system and can be used as-is or easily adapted for other games using the same core rules.