How to muscle a horse's neck ?

The strengthening of the neck comes with the use of their topline and lifting of their back. You have to avoid riding them upside down where they develop all the undesired muscle on the underside of their necks.
It will come very slowly overtime, they just have to be worked properly everyday, using their hind end, and stepping underneath themselves. If you fee like you aren't getting results fast enough, don't be discouraged. Remember it takes two solid years to fully develop a horses topline.
Several side reins in the long are possible : german reins, elastic, pessoa, gogue, chambon or long reins.

Put a slight bend to work in the right direction.

But by working the whole of your horse, it allows to bring him power and thus to strengthen his neck.