Answer:
Long and slinky like a trout, never sings till it's guts come out.
I run around the city, but I never move.
Forward I’m heavy, but backwards I’m not.
My first is in some but not in all. My second is into but not in tall. My third in little but no in big. My fourth in port but not in pig. My whole is made in nature's way. For clothing, rugs used every day.
One by one we fall from heaven down into the depths of past, And our world is ever upturned so that yet some time we’ll last.
You use it between your head and your toes, the more it works the thinner it grows.