1. A parasitic twin that protrudes from the body of certain beings usually taking sustenance from pieces of the hosts skin and bone. Typically, a twonster will be most prevalent at birth, and will consume the brother/sister host within twelve weeks. However, some lay dormant waiting for extreme sickness or long periods of malnutrition to begin the leeching process.
2. A large crick.
3. A peaceful creature that is trapped in the body of its dead sibling that was once evil and parasitic but is now filled with regret and wants to befriend you.
A twonster cannot be removed or killed for any reason ever.
I'd give you a back rub, honey, but that nasty twonster you've got on your back has a runny nose