I was thinking about what type of Hollow he was before an Arrancar. I apologise if this has already been thought of or it has been confirmed otherwise. I think 1-4 were Vasto Lorde, it showed Ulquiora when Uryu explained how Vasto Lorde always turn out looking human when they become Arrancars. 5-8 were Adhuchas and 9 was a powerful Gillian.
As we know Gillians are formed when a large group of hollows come together and start eating at each other. If one of those Hollows is able to control the Gillian it can become an Adhuchas. Most Gillians are however mindless. How does this sound? A large group of mindless Gillians were drawn together to consume each other. (possibly on there own, possible Aizen caused this) They eat at each other in the same way Hollows eat at each other to all become one Gillian. All these mindless Gillians eventually merged together into one mindless form. A giant hollow with incredibly high spiritual pressure but the same intelligence of a Gillian. This being was stronger than adhjuchas so started consuming them as well. As it was already formed the Adhuchas' minds did not take over the being, it's IQ did not increase but it's spirit energy and strength did, possibly it even consumed Vasto Lordes when it was powerful enough.
This being was made into the Arrancar which is Yammy.
That is what I was thinking. What backs up this theory would be that Yammy is certainly the stupidest of the Espada. He was given the tenth place because he would always be weakest of the ten Espada when sealed but in released form he would be stronger than any Vasto Lorde would be so became 0. Also he doesn't have anything unique to himself shown yet, (Apart from his ability to easily suck souls from humans with low spirit energy but they most likely could do that as well.) just Ceros and Ballas that most of the espada and Arrancars can do in addition to moves. Every other Espada has specials that only they do, like Aporro's ability to make the voodoo doll version of an opponent with that person's guts inside that if he destroyes them it destroys that person's guts. Yammy just has a strong version of the normal Arrancar abilities.
Does it make sense that he may be the result of countless mindless Gillians coming together to feed o each other and mergung into one in the same way Hollows become Gillians?