Menos (メノス, Menosu?, Spanish and Portuguese for "minus" or "less") Hollows have an unusually strong thirst for souls, leading them to consume other Hollows and evolve into a stronger state of being. Within the Menos are three levels of evolution, each with superior strength, speed, and intelligence compared to its previous form.
The first level of Menos is the Gillian (最下大虚（ギリアン）, Girian?, kanji translates as "Least of the Great Hollows") class, they are also known as "Menos Grandes" (大虚（メノスグランデ）, Menosu Gurande?, Spanish and Portuguese for "less big/not as big," kanji translates as "Great Hollow"). They are considered foot soldiers to the other Menos, and normally reside within the Menos Forest located below the desert surface of Hueco Mundo. The Gillians are powerful giants, masses of countless cannibalistic Hollow identities gathered into one form, each with very little intelligence or individuality.
If one of the Hollow consciousnesses that now make up the Gillian surfaces above the others, that identity dominates the entirety, and through eating other Gillians, eventually evolves to the next level, Adjuchas (中級大虚（アジューカス）, Ajūkasu?, kanji translates as "Intermediate Great Hollow"). Granted with superior intelligence, Adjuchas give orders to Gillians, and are better able to do combat with Soul Reapers. Adjuchas must continue to eat Hollows to maintain their individuality or they risk regressing to their original Gillian form. There are some cases where Adjuchas will halt regression, since if a part of their bodies are eaten they shall stop evolving completely.
The very few Adjuchas that transcend their limits become Vasto Lorde (最上大虚（ヴァストローデ）, Vasuto Rōde?, Spanish for "complete lord," kanji translates as "Great King of the Hollows") class. They are the closest Hollows in size to humans and have power greater than that of a captain-class Soul Reaper. At one time in the story, Captain Tōshirō Hitsugaya speculates that the Soul Society "would be doomed" if their enemy Sōsuke Aizen gets at least ten Vasto Lordes under his command. It is shown in the anime that at least one of the Espadas (Tier Halibel) was a Vasto Lorde before joining the Espadas.
Read more about this topic: Grimjaw