Link Anonymizer
http://community.mybb.com/mods.php?action=view&pid=684

Author: NewEraCracker
Branch: RELENG_1_8 for MyBB 1.8
Submitted: 24th September 2013
Last Updated: 29th June 2016

This  plugin automatically anonymizes the links in all messages posted
in your forum. Links containing forum domain or cookie domain will not
be touched, only external links will.

This  plugin  can  be  installed  easily.  Just  upload  the files and
activate it in AdminCP.