Well, it doesn't have an original mask. Originally, it actually used Arachne's own skull as it's focus, but fortunately the heroes who fought against the Black Beast managed to banish it back inside Arachne without killing her (Or crushing her skull). Later once Arachne get's better at controlling the Black Beast she can use her own skull again. 

Additionally, the Black Beast is [i]NOT[/i] under Arachne's command. Even if she calls for it and it's awaken, it won't come out unless it wants to. And it'll only come out when it can find a face, i.e. a skull-like mask. So if she ever wants to use the Black Beast for whatever reason, she needs a mask.