I stumbled across one in a random progression fantasy series a while back, and the concept kind of latched onto my brain. It wasn't even the main character, just a side companion, but the sheer versatility stood out. They're not just a fox, and they're not just slime. You get this weird, wonderful hybrid that can mimic shapes, absorb materials, and sometimes even memories or magic from what they consume.
Unlike a standard shapeshifter, there's a gooey, permeable quality to the transformation that authors have fun with. One book had the slime fox form protective bubbles or slippery traps, which was a neat tactical twist. Another played up the sensory aspect—the fox could extend pseudopods to feel vibrations or 'taste' the air, which made for great exploration scenes.
But what really makes them distinct, at least to me, is the tonal range. They can be adorable, almost pet-like companions one moment, then this eerie, otherworldly entity the next. That duality opens doors for some really interesting character dynamics, especially if the fox is sapient. Are they a loyal friend or a parasitic symbiote? The line gets deliciously blurred. I'm still waiting for someone to write a full-blown horror fantasy with one as the protagonist.