As it happens, Liv Breeden, one of the HS devs, has just been clarifying this in Reddit:
"Zayle will not show up in packs. There are 5 Neutral Legendaries, not including him. You get the golden version in the single player content, but if you want the normal one, you have to craft it."
And also: "he won't show up in the collection until the single player content releases."