Re sensory: From what I've read, the dreaming brain takes advantage of many existing brain-structures - eg, when you see things, parts of the visual cortex light up. So it seems plausible that that would also work for physical sensations.

I suppose one possible resolution is that my particular sensory homunculus is already primed for a tail somehow, though, again, I'm not sure how to find evidence to help support or reject that theory.