Presets are all pretty much just a modified texture(s) slapped onto an already existing body mesh, which in this case is the canine or feline body. No one would be able to see what you see if you changed or had different meshes and then a texture put onto that, instead it would look completely deformed if you actually went through all the work to replace certain meshes in the game now with something of your own.
So to my understanding it's just a regular preset put onto either a feline or canine body, and then modified as much as possible to look like a fish. Even Kiki's preset had an empty transparancy liny going through if you looked from the front. So to my understanding they used transparancy on some part of the preset texture and made a fish like look on another part, making it seem like a fish.
If you want more detail on how to actually making a preset like theirs it would probably be best to ask them.