They are also known as Susuwatari/Makkuro kurosuke in Japanese, and Wandering soot/Pitch-black assistant in English.
Susuwatari are described and shown as tennis ball-sized, pitch-black and fuzzy-haired beings with two large eyes and long, thin legs. They are said to live in old, abandoned houses and cellars and they love spreading around dust, soot and dirt. Susuwatari can carry things that are much heavier than themselves. In My Neighbor Totoro they scare the girls Satsuki and Mei by accident, but remain harmless and shy. In Spirited Away they are the servants of the spider-yökai Kamaji and they have to work hard for him.
I've made a whole bunch of sottroll. They are so cute. They like the attic and any dark room. They heavily foul the house. So I have hidden them around my house, especially in the guest room!
I made them to prank my sister :P