Yeah, but it was nearly 10 years ago, with a very different art style ! Actually i've been thinking about including them in the Éphéméride from the beginning.
Charybdis can be fun to design but i have to put some thought on it to find something clever. Scylla has many canon designs, the one with the dog heads could be great for the Éphéméride but also very tricky to draw with the small canvas of the serie. It's the kind of monster i would like to have several takes on !
At some point i tried to include every past monsters i've designed into the Éphéméride, so my old version of Scylla has been done (n°245). The idea was that Scylla is a giant polyp, and their offsrpings nourrish Charybdis. I also tried to redesign Scylla, with more focus on the anatomy of the organs : their organisation shows how they derived from human anatomy with many duplications and deformations (the toxic tentacles with "feet" are actually extra long salivary glands lol)
(and the detailed sketch for the head is based on gorgoneions, because gorgonian corals -> polyp was something logical for me aha)
So yeah ! I've been thinking a lot about Charybdis and Scylla, and have hidden some related ideas in the Éphéméride or recent sketches. But because i love them so much and they can have so many different designs it's difficult for me to settle down for one