The small human made her best to hide while the demon collected his belonging and made his way out of the crater. Rin followed her original plan of staying behind chasing Sesshoumaru from afar. She also had already plotted to justify her lateness due to having fallen asleep next to a lake and on returning having tumbled on a mud pit, hence her dirtiness.
After a few hours and just like before, Sesshoumaru had returned to the camp, Rin went with her plan which seemed to have worked out since neither of the demons questioned her, simply ordering her to clean herself especially since she carried the smell of the entire forest with her.
As the girl washed away the mud and soil she kept thinking about Unkoyasha and his relation to other powerful demons such as Moryoumaru or Naraku, or less powerful demons like Shippou or Jaken. Her pensive moment could have taken a whole night's sleep and would have led to nothing, so instead of wasting precious resting time in a matter which could not be solved by over-thinking it, little Rin snuggled down sighting with deep relief that next year, she would have nothing to worry about.
Of course the grim face of demons, or people, is always a harsh matter to wrap onto and so it is often left unspoken and many questions bump readers mind of what these dark sides are like and what could they bring or take. Being the wise old demon I am, I have anticipated most of your questions about Unkoyasha, but of course have to disapprove of your nosing with demon-only business, but since you've gotten this far, I should as well praise and appreciate your patience and interest for this short story.
The reason why Unkoyashas exists is because they are infused with demon energy since they are formed inside the demons' body. Sesshoumaru only goes for such a walk around once per year because he, like other demons, doesn't eat that much, neither that often so most of the digested food is in fact absorbed by his system. What his body disposes of is waste and unwanted demon energy, resulting in the formation of an Unkoyasha.
All demons must slay their own faeces upon expelling them because the demon miasma embedded in the poo is poisonous to all other living being except its own creator, this means Naraku for example, could take advantage and form a massive Unkoyasha army that would surely destroy the world. Fortunately, Mother Nature has yet again been wise, and has made sure that Unkoyasha's first victim would be the one who created it, and if a Unkoyasha is left unattended it will grow powerful, more powerful than the original demon. The crapped demon will then seek its maker in order to bring judgment by penalty of death. In case the creator dies, Unkoyasha will also die, it will be left to dry like any other excrement and consumed by nature. To prevent sure death and shame of having being killed by your own faeces, demons slay their creations immediately after having soiled.