I am thinking of a story where a meteor carrying a microorganism lands into some hot spring mountain caverns that between a city and a desert and evolves to be able to able to take DNA samples from other creatures and combine them to create a new creature that pretty much uses the microorganism as it's blood. The creatures thou different share the trait of having two stomachs, separated by a interchanging throat. One with strong acids that breaks down organic matter for substance and one that is filled with the microorganism that takes the DNA info from parts that the creature thinks might be useful for it's offspring for the reproduction cycle. The creatures live in a cycle of seeing each other as a potential meal/DNA to having only reproduction on it's mind. The reproduction cycles starts with 2 creatures "throwing up" the contents of the microorganism stomach into a pile. The pile quickly forms a thick outer coating as the DNA is mixed together in goo, making a new batch of monsters. By the time of the story the biggest creature which is up to 30 feet tall on it's hind legs while most are 16 feet tall or smaller, has caused the mountain to erupt open, bringing the creatures to the city. I am thinking the reason for the biggest creatures growth is due to consuming some of the original microorganisms in the meteor. What do you think would a reasonable amount of time for the creatures to grow to this size, how many creatures would be formed on average per goo? How often the reproduction cycle would activate? Do you have better idea for the biggest creature's growth? Would the creatures be noticed by many people before the mountain breaks? I hope this all makes sense and I hope you have a great day!
Make it awesome.
If you want this to be biologically accurate, monsters need to eat something to grow. If they have a lot to eat they can grow faster. There is not much to eat in most caves. They might not grow too fast.
But I like your monsters. Maybe they can grow on the power of radiation, or emanations from some weird pool deep in the mountain. Or just be monsters!
I am reminded of The Seven Geases by Clark Ashton Smith.
Here, it seemed, was the ultimate source of all miscreation and abomination. For the gray mass quobbed and quivered, and swelled perpetually; and from it, in manifold fission, were spawned the anatomies that crept away on every side through the grotto. There were things like bodiless legs or arms that flailed in the slime, or heads that rolled, or floundering bellies with fishes' fins; and all manner of things malformed and monstrous, that grew in size as they departed from the neigbborhood of Abhoth. And those that swam not swiftly ashore when they fell into the pool from Abhoth, were devoured by mouths that gaped in the parent bulk.
Finesse the biology. Monsters! Have the desert people around the mountain disappear. Or one comes into town raving about monsters, and unlike prior episodes he would not settle down when offered booze. Someone decides to go check on company operations in the area and finds the work site deserted and signs of things having been dragged back towards the mountain. They investigate...