What were the costumes at the masquerade ball?

The costumes are many and varied, and are never identified individually. Yet Poe's description creates a scene of revelry, where costumers let their minds run free to design masques of all imaginations.

Be sure they were grotesque. There were much glare and glitter and piquancy and phantasm—There were arabesque figures with unsuited limbs and appointments. There were delirious fancies such as the madman fashions. There was much of the beautiful, much of the wanton, much of the bizarre, something of the terrible, and not a little of that which might have excited disgust. To and fro in the seven chambers there stalked, in fact, a multitude of dreams.

Thus, the costumes were anything those in attendance could imagine. There were shiny and glittery costumes, and ones with too many arms/legs. There were people dressed as madmen, and people dressed beautifully and terribly. Basically, if you could think of it, you could wear it.