So, I found a poll about “miniboss encounters”.
https://undertale.wiki/d/t/favorite-miniboss-encounter/1192
What’s interesting is that every single entry is one of the ones I don’t consider minibosses.
I’m not sure what the rules determining encounters are, but maybe saying minibosses specifically ARE NOT encounters is a way of narrowing things down? I’m not sure what criteria was used to determine “encounters” but I also consider anything that’s just a random encounter to not be a miniboss.