Broken Hollow Masks?

It does and Arrancar is a hollow who has broken the barrier between Hollow & Shinigami, and what the Hokyoku does is exactly that it breaks the barrier between the two sides.

So far from whats been 说 in the 日本漫画 the Stark the Primera Espada has been the only Arrancar shown to have reached Arrancar state without the Hokyouko. I'm am positive there have been other Arrancars before him but so far he's the only one that Kubo has shown.

From what I've understood Arrancars usually is a state that a Hollow reaches after it has evolved in a Vasto Lord (the 3rd and final Menos lv.), it probably takes years to achieve this level decades possibly centuries but nothing else is really known.