when I was 8 years old my mom's boyfriend put his private part(Dick) in my butt .He said if I told anyone He will Kill my mom but after that I told my Little brother and he told her my mom called the cops and hes arrested Did I get Raped?

Yes. Any unwanted sexual penetration is rape - this person acted unlawfully, cowardly and you were right to tell your brother and mother. He should definitely have been arrested.

I hope you know that being raped or sexually assaulted is not your fault - you didn't "ask for it", you didn't deserve it and you are not a bad person or less of a person because it happened. I hope you're okay and that you have the support to move on with your life and take back control.