If you want rxvt-big5 to automatically detect multibyte characters when
moving the cursor, put the following line into your .Xdefaults or .Xresources:

rxvt.multibyte_cursor: yes

If you do not wish to have this feature, reinstall the port with the following
commands:

cd /usr/ports/chinese/rxvt-big5
make NO_MULTIBYTE_CURSOR=yes deinstall clean install clean