Ok, here's what I could find:

According to SauceNAO, it appears to be made by a DeviantArt user called Spray-POKA, to commemorate the release of Pokemon Super Mystery Dungeon in 2015. However, they have since deleted the post.

The artist draws lots of transformation art, especially comics, and the characters in this piece seem to be transformed versions of the artist's OCs.

• Some of the artist's OCs can be seen here. Katori is the Frillish (note the glasses, anchor, pipe), Jiroro is the Florges (note hair, glasses).
• The Chesnaught, Frillish, Krookodile, and Zoroark appear in another piece that the artist made for halloween 2015. The Chesnaught seems to be a TF of the artist's sona.
• The Florges appears mid-transformation here.

I could not find evidence of a full-on fanfic or long-form comic about PMD. There's a bunch of short TF comics and one-off pieces, though.

Note from uploader: The source and artist of this image are unknown; I could only find secondary copies here and here. I assume this is promotional art for someone's PMD fanfic. Any help tracking down where and who this image belongs to will be much appreciated!

swinging said:
Ok, here's what I could find:

You rock!! Thank you for the detective work. I could not make any headway on this.

Bummer that the artist deleted the piece, and that there isn't a longer work behind it. The TF comics are well-drawn, but good Lord that's some seriously broken English...