

The hit dice for a monster in 5e are basically completely arbitrary. If you’ve come from previous editions where the total number of HD was equivalent to the monster’s level, that is no longer the case.

If you haven’t looked into the monster math for 5e (starting on page 273 of the DMG), your intuitions about altering HP might be wrong. Meanwhile, GMs often figure that if they just set a monster higher in its HP range (by default, they take the average dice rolls) that’s still fair but might make the fight a little more challenging. One of the classic ways to put a higher-difficulty monster into a module for lower-level characters is to have it be already injured before they fight it, giving it fewer HP.
