I would like to expand this list with as many new fitting entries as possible. I suppose a 1000 would be a nice number to look at.
Originally, I had around 200 or so characters that I just came to know organically through... being around, I guess, but after time, I did try to think of efficient ways of finding new monster girls.
The most effective method by far was a script that used the Rule34 and Gelbooru APIs to fetch characters that co-occur with specific tags, applying a minimum ratio cutoff and sorting them by post count. I worked through tags like mermaid, lamia, dragon_girl, demon_girl, feathers, tail, and so on, and then reviewed the resulting character lists.
I also manually went through the monster_girl tag on Rule34, Gelbooru, Pixiv (モンスター娘 OR 人外娘), e621, a few doujin sites, some anime and manga sites that provide such tags, and similar.
Now that I've reached this point, I'm unsure what other worthwhile methods might still be available. If anyone has any ideas, I would be grateful.