Hooleer

Superchunky is a large creature that appears to be fighting alongside Sōsuke Aizen. It appears to be a Hollow of some description, with a bulbous body and a large single eye. It also has the masks of Gilian class menos like-toenails. It made a cameo appearance early in the series, but recently made its proper debut during the winter war. It can stake a claim to being arguably the largest creature yet seen in the series.

Soul Society arc
It made a short appearance as Aizen, Ichimaru and Tosen were being extracted from Sokyoku Hill via negacion. Its eye can briefly be seen (though the viewers do not what it truly looks like at this stage) behind the mass of Gilians present within the Garganta.

Fake Karakura Town
Wonderweiss Margera appears during the captain's battle with the espada, bringing Superchunky with him. It is evident that it dwarfs the other combatants. While Wonderweiss attacks Ukitake, Superchunky uses his breath to extinguish the fire prison that is trapping Aizen, Ichimaru and Tosen, thus freeing them.

Powers and Abilities
Breath: Superchunky's breath is powerful enough to put out the fires caused by Genryūsai Shigekuni Yamamoto's zanpakutō ability, Jōkaku Enjō (though it must be said that they had already been burning for some time at this point). It is implied that it 'smells of death'. Whether this means it has other more obscure effects remains to be seen.