I'm not part of the discord but Obstetrics informed that it was decided there that Pokemon creatures prevent an image from getting a solo tag even when there's only one human character in it. I find this counterintuitive because it goes against how animals are usually tagged. Now, not all Pokemon are animals, humanoid ones like Mewtwo, Gardevoir, etc, are humanoid enought to qualify for at least a 1other tag. But the vast majority are in fact just cartoonish animals, so typically they wouldn't be considered humans for character count tags (unless their genitals are showing, which is a different matter altogether).
If one were, for instance, to search for rosa_(pokemon)+solo, they would expect to find every pic with Rosa as the only human. But if one wanted to search for pics of only a non-humanoid pokemon, they would use no_humans instead of solo (creatures don't have counters so it's hard to search for pics with a single species of Pokemon). Why would a search for rosa_(pokemon)+solo miss posts like post #4486642 because of some lizard?
This is why I think animal Pokemon should not affect a solo tag. Danbooru is overwhelmingly focused on human character art, so it seems silly to me that Pokemon in particular are given special treatment when it comes to counting them as characters.