Do monsters rolled with 0 HP get a minimum of 1 HP?

Question

When rolling HP for monsters, is 1 the minimum? Take the Gas Spore for example:

Hit Points: 1 (1d10 - 4)

What happens when a 4, 3, 2, or 1 is rolled on the d10, especially when said creature explodes violently when it reaches 0 HP?

Answer 1

1 is the minimum

Jeremy Crawford tweets that the intended minimum hit points for a given HD is 1, not 0. So your gas spore would have 1 hp.

But it doesn't matter

Although when I get "fencepost" numbers on rolls I use this as an opportunity to liven things up. The world exists other than just for the PCs! If I were to roll a creature with 0 hp, it might be there but dead. The others of its kind killed it, or their enemies killed it - plot opportunities! Imagine instead of "you come across six goblins, they attack you", "you come across five goblins kicking another goblin to death" or "you come across five goblins standing sadly around a fallen fellow." Or for something just like a gas spore, they find the remnants of an exploded gas spore, providing a clue that there may be more around... You have de-lameified a low level encounter.

For the DM, even if you feel obligated to always go with the die roll when directly engaged with the PCs (in combat etc.), die rolls in other circumstances are purely to help you and spur your imagination. Interpret them how you want, use it to add flavor and give you ideas... Feeling bound to rolls when setting up adventures goes beyond RAW to OCD. Replace the monster, reroll its hp, set its hp manually, or do something more interesting - it's your game.

There is no expectation, even in RAW play, that DMs abide by die rolls during adventure setup.

Answer 2

It doesn't matter

Since this is for monster HP, which you will presumably roll for before you present it to the party, then if you end up with a monster with 0 HP or less, just roll it again or take the average.