Hollows

Hollows could be categorised in these ways

Demi-Hollows: These are ones who are in the midst of transforming into a hollow after their chain of fate has partially corroded

Low class hollow: A just transformed hollow

Mid-High class hollow: The hollow has gotten stronger due to how many souls they've eaten.

After that comes the menos, which are basically the evolution of a hollow when human souls are not enough to sustain them. So hollows then resolve to cannibalism where a massive inter-devouring follows which then creates a Gillian

Gillian are huge creatures that are a few stories tall and have beast like intellect. They have their own abilities and are far stronger than normal hollows but are the weakest of menos.

After that come the Adjuchas who are like Gillian's but have a higher level of strength and hogh levels of intellect.

Lastly, there are Vasto Lorde who are the third classification of menos and, by far, the strongest. They are extremely rare and rarely leave Hueco Mundo.