I am aware there's documentation for Improving Monsters but I was curious if there's documentation/rules/guidances for making weaker monsters than their default stat blocks.
Specifically, I was mulling that many monsters grow (are not created immediately with the stats (specifically HD) in the stat blocks), for easy (and real) example: humanoids and animals. They could be in a combat either designed (to go easier on lower level parties) or because they exist and are attacked.
NOTE: I could make up rules for this, but am curious if there are existing ones; I don't desire someone's homebrew solution.
Another answer I was mulling was that younger versions just aren't meant to be in combat. I actually recall (but did not find, nor am I sure if it was a rule or just author discretion) a module where a group of lizardfolk children (and women!) were non-combatant (no stat block) and there was a moral dilemma if the party should just slaughter them or leave them alone. This is especially interesting to me because the typical lizardfolk is 2HD so there (conceptually) easily could be a "weaker" 1HD version.
I think for another example, a baby hill giant could do some damage to a level 1 character. Even kittens can scratch.