More monsters should undergo metamorphosis. Let’s consider some examples, ranging from the canonical, to the reinterpreted, to the brand new.
Piercer to Roper
Some mock the piercer, alleging that its behavior and life cycle is implausible. But I think it is a great monster and makes a lot more sense when you emphasize that piercers grow into ropers. Many animals reproduce by creating many spawn, expecting only a few to survive to adulthood, so a cavern full of piercers in roper territory fits. The piercers also complement the ropers’ tactics; a roper dragging an adventurer toward its maw provides an easy target for some piercers to dive bomb from above.
It always irks me that the 2014 Monster Manual puts the Roper and Piercer in separate entries, while the same book groups the following two creatures in one entry, despite similar immature/mature growth stages…
Fire Snake to Salamander
Fire snakes have some good flavor (“When a salamander is ready to hatch, it melts its way through the egg’s thick shell and emerges as a fire snake” – that’s good!) But it doesn’t do much to explain how it turns into a salamander. It also is pretty unremarkable as an encounter, with “hot stove, do not touch” as its entire personality.
What if a fire snake consumes heat to turn into a salamander? No big deal on the elemental plane of fire, where heat is plentiful. But on the material plane, it gives them a much more compelling hook for conflict, as they gobble up fires of all shapes and sizes to fuel their transformation.
Gas Spore to Myconid
The lowly gas spore is another target of mockery for its work-backward-from-dungeon-logic ecology. I don’t think that’s a problem per se, but we can fix it with a non-canonical hack. Instead of assuming that gas spores just produce more gas spores, why not make them the source of new myconid colonies? Either as an intended transition from one life stage to another, or perhaps as myconids hijacking the unintelligent gas spores to spread their colonies?
Lurker Above to Trapper
More gimmick monsters from the early days of D&D. These guys have gradually lost ground (or ceiling, respectively) in the game’s cultural consciousness to mimics, which have escaped the confines of D&D to become a fantasy staple (in Dark Souls, of course, but also prominently featured in manga and anime like Dungeon Meshi and Frieren). Thinking about such creatures makes me wonder – does the existence of the executioner’s hood, darkmantle, and cloaker imply that there is some evil pants-mimicking creature out there that can complete the aberrant wardrobe?
Regardless, I do like the implicit idea that camouflaged ceiling monster and camouflaged floor monster might be two parts of the same species' life cycle. Looking at the original AD&D Monster Manual, they’re awfully similar in most respects, but the text doesn’t make the connection explicit. I believe the lurker above is the juvenile form and the trapper is the mature specimen.
Why? Because of my favorite detail distinguishing them – the lurker above is non-intelligent, but the trapper is “highly” (!) intelligent.
You, a pleeb, clinging desperately to the ceiling. Me, an intellectual, resting comfortably on the floor.
Caterkiller to Butcherfly
Here’s an original idea for you. The caterpillar and the butterfly are probably the most famous example of metamorphosis in nature, so let’s monstrify them. The caterkiller is a slow-moving meat shield that loves to consume paper and textiles of all kinds, whether that food is garbage, treasure, or adventurer apparel.
Coming into conflict with adventurers actually helps trigger the metamorphosis. The “blood” it bleeds from its wounds hardens into a cocoon. Leave a “dead” caterkiller behind, and you will find a pupae when you return. Wait too long before you return, and that pupae will have already split open. A butcherfly now roams the local area, every bit as nimble and savage as the caterkiller was slow and methodical. Remember to tell the PCs the monsters' names. Both forms should behave like tokusatsu monsters and should probably do menacing poses when the PCs first encounter them.
