I think most of the harpy types are overlooked?
Harpy as mentioned before, Black Harpy for having a really strong familial protection, siren as a singer, Owl Mage as being smart and capable of magic (this is one of my more favorite monsters) I'm actually super surprised most people don't bring up the gandharva, a wandering musician/muse type monster, the crow tengu is another one that I think has a lot of potential for being solitary and studious and only showing up during mating. The cockatrice is smol, so I'm not surprised people don't speak much about her. But she could be interesting. I feel like the jubjub is the most talked about harpy, on account of being everywhere in wonderland
I skipped over the thunderbird because besides her there's also the raiju. I don't quite care for either of the lightning elemental mamono, I wish there was a third option, but they both seem to have merit, and the raiju has all the hallmark behaviors of any zipangu mamono
Cupid and Houri
Dark Priest, which I kind of like for some reason
I still feel like I'm somehow missing the appeal of the Kesaran Parasan, including what she's supposed to be from. Little dandelion looking girl that drifts in the wind and like makes you high from her spores or something
I have commented before that I think Kraken always gets put on shipwreck duty (though I do her very much) and never leaves room for the charybdis, which could also allow you to bring in the scylla