In the real world, the idea of spontaneous generation, where maggots and flies come from rotting meat was disproved and replaced by the actual method of like creatures spawning like creatures. That is called biogenesis. [Catch the companion podcast here.]

However, in a fantasy world, spontaneous generation can be real.

So normal creatures that die spawn maggots and flies.

Monstrous and magical creatures would spawn something more fantastic.

For example, if a dragon is slain, suppose the creatures spawned from it are carrion crawlers and purple worms.

Not ever creature needs to do this, but once you have one creature do this, if the players make the connection, they will burn every dead body ever encountered. Perhaps some creatures only spontaneously generate from the ashes of their cremated bodies.

Some creatures will have more rapid spontaneous generation than others. The more powerful a creature, the more rapid and impressive spontaneous generation could be.

There are a couple of ways I see of handling this.

One option is to specify a certain type of 2 or 3 creatures that are generated when a specific creature dies.

The other option is to generate a table of the kinds of things that can be spontaneously generated. Perhpas there is only a chance of it happening, or the onset of the new creature is variable, etc.

It occurs to me that this is one way a seemingly inaccessible dungeon manages to re-populate.

Perhaps only certain creatures have this happen.

I can imagine a table where after d6 minutes or hours the creature either dissolves into slime and sludge or blows away as ash in the wind or generates 1 to 3 random unrelated creatures. This limits the efficacy of collecting parts. Get them now or they won’t last long enough for you to come back.

This is an idea to keep in the back of your mind for when weird things get mixed together. Like when the players mess with the experiments on the wizard’s workbench. Perhaps some horrid beast is created by messing with wizard’s experiments. It could be some “regular” monster, or some hideous creature created by the 1e DMG table for randomly generating creatures from the lower planes.