I commend Wendy on her ability to draw a diagram in
ascii, but I would like to describe a method to tie
this knot in a way that will not tighten like a noose
(and Wendy, maybe you can make a diagram for us?)
Following is the knot as tied by the RCMP, U.S Cavalry,
etc. (according to my 80+ year old buccaroo friend.)
Holding the lead rope out from where it is attached to
the horse's halter, about 1/4 from the attachment, make
an "N" in the rope in your hand. The right leg of the
"N" continues around the horse's neck, and back to the
"N". Bring the rope through the upper left "crotch" of the
"N". Next, gather all three legs of the "N" together in
one hand and, with the lead rope end, make several wraps
around them until you come to the bottom of the "N" and
the end of the lead rope. Tuck the end into the lower
right "crotch" of the "N" and pull everything snug.
This knot will appear the same as a hangman's noose, but
will not slide and tighten on the neck.