Hooleer

This Giant Unnamed Hollow is a large entity that appears to be fighting alongside Sōsuke Aizen. The true nature of this creature remains unknown, but it seems to have a connection with Gillian-class Menos, suggesting that it is indeed a Hollow.

Character Outline
It has a bulbous body, long tufts of hair on its back, and a single, large 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 fingernails. It had a brief appearance early in the series, but recently made its proper debut during Aizen's invasion of the Fake Karakura Town. It is arguably the largest creature seen in the series yet.

Soul Society arc
It first makes a short appearance as Sōsuke Aizen, Gin Ichimaru, and Kaname Tōsen are being extracted from Sōkyoku Hill via Negación. Its closed eye can be briefly 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 in terms of size. While Wonderweiss attacks Jūshirō Ukitake, the Hollow uses its breath to extinguish the fire prison that is trapping Aizen, Ichimaru and Tōsen, thus freeing them. After Wonderweiss screams, it releases Gillians from his mouth to attack the Vizards. After the Vizards kill the last Gillian, Mashiro Kuna attacks it with her "Mashiro Super Kick", seemingly killing it, while Wonderweiss simply stares.

Powers & Abilities
Breath: The Hollow's breath is powerful enough to put out the prison of fire made by Genryūsai Shigekuni Yamamoto's zanpakutō's 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: It is shown to be able to summon numerous Gillian Menos from his mouth. At first, it looks like a mass of black liquid, until the Gillian Menos separate into their individual forms.

Appearances in other Media
This giant creature appears as one of Aizen's special moves in Bleach: Blade Battlers 2nd, where he spits out Gillians to attack.

Trivia

 * While the real name of this Hollow is still unknown, many fans refer to it as Superchunky because it was first fully introduced in the chapter titled "Superchunky from Hell".
 * Beyond having Gillian masks as fingernails, this Hollow does not appear to have a mask.