Answer:
the only amusing part of the body
Lovely and round I shine with pale light grown in the darkness a lady's delight.
A trail, a union, together tied. Come across me and you will find you cannot change the course I'm on. Without me you cannot travel on. What am I?
I walk on 4 legs in the morning 2 legs at noon and 3 legs at night.
Ripped from my ancestors home beaten and burned I become a bloodthirsty killer.
A little house full of meat, no door to go in and eat.