There two basic types of rope. Static and dynamic. Dynamic rope is made to stretch a good bit. It is used for rock climbing and mountain climbing a good bit for fall protection. It will stretch and not just slam someone to a stop or worse yet just snap from being shock loaded. If your halter is stretching alot you may have gotten a dynamic type rope.
I need to learn how to tie some halters up. I'm decent at knots just haven't had the time to sit down a figure it out.