Any objections if we just get rid of monsters?
Add a wiki and keep the tag
Not being a popular tag should not be ground for deletion. The three question that use it are clearly related and no other tag would aptly describe how or why they are related. Additionally there at 66 questions that use the word monster but don't have the tag. I'm sure at least some of them would benefit from it.
Here is a suggestion for the usage guidance:
For questions about things that go bump in the night. Questions about real or imaginary monstrous creature, how to describe them or incorporate them into works of fiction.
And a tag wiki:
Monsters are a staple of the horror and fantasy genres, but also appear in children's literature and various other works. From the monster under the bed to the bogeyman, this tag covers any creature that could be described as a monster.
Questions with this tag should focus on the narrative elements of including a monster in a story, or the physical description of monsters. Questions about creating monsters are off-topic. Some example questions:
How can I make the monsters in my children's story less scary?
How do I make a monstrous character relatable?
What are the advantages of keeping the monster unnamed in a horror story?
From those suggestions you should be able to see what I think this tag should be used for. I can see a clear use-case and don't think it needs to be removed.