In the most recently-scanlated chapter of Fujoshi Kanojo, Yuiko and her fellow fujoshi were sighing over a "tuna uke" series where a fish turns into a man and falls in love with a human. Taiga was, of course, nonplussed, but I was interested. XD

So I'd like a series like this. A human and a something-else in human form fall in love. Does it exist?

That scene in Fujoshi Kanojo cracked me up so much.

It certainly exists:
Glamorous Lip -- the story "Ma Cherie"
Junketsu Ningyo -- merman, beetle, snake...
Sex Pistols

Oh... reading your request again, I don't think my suggestions fit because they're not tragic. But still try Junketsu Ningyo -- there's a story about a pet beetle that falls in love with its owner, and I had the exact same reaction as Taiga when I read it. 🤣

EDIT: I found a better example: the oneshot "Fairy Garden" from Bond(z). It's a tragic romance between a gardener and a flower.

Also, erm, this thread might help.

Avatar by tassyn.livejournal.com

Reading: Kingdom, Sangokushi, Historie

This thread might help. ^^

For tragic, all I can think of is Innocent Bird which you've probably read.
And Mister Mermaid that I haven't read myself~

And also (not sure if you've read), the very sweet:
Renai Chuudoku that features bugs. :3

Hrodulf and Bjornolfr, you will not be forgotten.
[img]http://i411.photobucket.com/albums/pp199/SyberAngelGabrielle/couplesig.jpg[/img]
[color=black]And if the world were black and white,
you would be my rainbow in shades of grey.[/color]

Click 'n Play!

If I had a fantasy self, it'd be a tentacle monster.

Love Neco
Hybrid Child <--The only tragic one of these three.
Both deals with human-like dolls.

Oneshot:
Hara Peko Mitsuba Chinko (Human x Plant)