If you wrote a short story about a monster, there's some chance that the monster exists, there are legends about it, one of those legends made its way to you, and you then chose to use it in your story. Thus, if I never heard of the monster, then you wrote such a story and I had no further information about your story-writing process, that would indeed increase my estimate that the monster exists by a miniscule amount.

If you wrote the story specifically to prove a point in a lesswrong discussion about monster names that are pulled out of thin air, that would be further information, so my estimate wouldn't increase in the same way.

Moreover, it's not twice the evidence anyway, since the story and the lesswrong post aren't independent.