Hooleer

This Giant Unnamed Hollow is a large entity that appears to be fighting alongside Sōsuke Aizen. It is actually still unknown what this creature really is, but being that it is connected with the hollows (more than others, the Gillian class Menos), it most likely is one of them. It has a bulbous body, long tufts of hair on its back and a large single eye. Its hair appears to be the same or a similar color to its skin. It also has the masks of Gillian class menos for toenails. It made a slight 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 makes a short appearance as Sōsuke Aizen, Gin Ichimaru, and Kaname Tōsen are being extracted from Sōkyoku Hill via Negación. Its eye can briefly be seen behind the mass of Gillians present within the Garganta, but the rest of its body remains hidden.

Fake Karakura Town arc
Wonderweiss Margera appears during the Gotei 13's battle with the Espada, bringing the creature with him. It is evident that it dwarfs the other combatants. While Wonderweiss attacks Ukitake, the Hollow uses its breath to extinguish the fire prison that is trapping Aizen, Ichimaru and Tōsen, thus freeing them.

Powers & Abilities
Breath: The Hollow'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.

Menos Manipulation: He is shown to be able to summon numerous menos from his mouth. At first, it looks like a mass of black liquid, until the menos separate into their individual forms.