For questions relating to either monstrous antagonists specifically, or all enemies of the player characters generally, depending on the traditions of specific games. Use as appropriate for the game in question.
“Monster” is a term that can be used (as in the D&D tradition) to refer to any opponent of the player characters, including enemy humans; or can be used to refer specifically to monstrous creatures, whether hostile or friendly (as in a variety of other terminology traditions). This tag covers both usages and should be applied where it makes sense for the game, question, and campaign in the question.
For some previous discussion of the tag’s usage, see The "monster" tag – should it be broader? *Is* it broader?