An example of a social animal with a truly egalitarian social structure?

The bonobo is a well known example (at least in zoological circles) for 'some' of what you state. Egalitarianism in its pure, applied-to-every-aspect-of-social-life form, I don't believe exists for any social animal, bonobos are at least noteworthy in this regard. Likewise for social hierarchy/organisation.

http://en.wikipedia.org/wiki/Bonobo

As it happens, I just read the social section from the link above, it is poorly written and incomplete in my opinion, I suggest you have a good look online for a more complete understanding of their society.

There is no social hierarchy in a large school of fish, for example, herring. Keep in mind that there is a difference between social and gregarious. A group of migrating lemmings is not a social gathering because they do not interact with one another, and there is no social structure. They are just there in large numbers at the same time because they were all driven by the same urge to migrate because of overcrowding.

Human society is another example. In the beginning, when we first ventured into the savannah, there was a noticeable decrease in the size of the canine teeth, seen in the oldest hominid fossil Ardipithecus, indicating reduced male-male combat and increased male-male cooperation. A concurrent increase in penis size indicates that no single male has a monopoly on the available females within a group, and that the only way to ensure mating success is to have penis that is longer than other males in the same group in order to put the sperm closer to the uterine opening of a female. In contrast male gorillas have a tiny penis of 1.5 inches because he does not need to do what early hominids did: compete sexually with other males, since the gorilla social group consists of a single alpha male and his females.

From an egalitarian beginning, we have evolved culturally into settling down in villages, cities and now countries. As our social group grows in size, some human beings attempt to attain alpha status, and one of the ways they do that is through military might and another is to claim that they are gods. Egyptian pharoahs and Chinese emperors alike claim that they are living gods. Such a claim is often not enough to ensure that the emperors and pharoahs will retain their elite status, however, as rebellions are common in history of both cultures. Because humans are egalitarian in nature, we are seeing challenges to the rulling elites in autocratic countries such as Burma, Egypt, Tunisia, Libya and Syria. Because the elites in these countries have a difficult time justifying their elite status, they had to resort to using force when their privilege status is challenged.

Schooling fish come to mind. And swarms of birds, bats,...