Although "the Enneagram" is most often used to refer to a system based on nine personality types, "the Enneagram" is actually a symbol that was introduced to the world by G.I. Gurdjieff around 1916. Gurdjieff didn't use the symbol to represent personality types. He used it quite differently. However because Gurdjieff is associated with the symbol, people often mistakenly associate him with the personality types as well.
During the 1950s and 1960s, Oscar Ichazo began using the Enneagram symbol to represent what can be called "ego-types." He didn't use the symbol in the same way that Gurdjieff did but started a new use of the symbol. The purpose of Ichazo's enneagrams was to identify how an individual was fixated in ego and how that individual could find some freedom from that fixation. The enneagrams were used in the context of a larger training system taught in what came to be known as the Arica School.
In 1970, a psychiatrist named Claudio Naranjo learned about Ichazo's enneagrams from Ichazo himself. Naranjo used them in his SAT groups and evolved them into the personality types used today (he called them enneatypes). As people learned of the enneatypes and Ichazo's enneagrams from Naranjo, they began adding their own interpretations and concepts to the personality types and ego-types.
It's sometimes said that the Enneagram personality types are an ancient system for understanding personality. That's not quite right. The various people who evolved the system, beginning with Ichazo, simply added concepts and interpretations from traditions they were familiar with. It would be more accurate to say that elements of older traditions can be found in the current system. What elements are found often has to do with whose interpretation is being looked at.
It's important to note that the symbol and the types have two separate histories that don't overlap. The history of the symbol goes back to G.I. Gurdjieff and the Sufis (from whom Gurdjieff purportedly learned of the symbol) while the history of the personality types goes back to Oscar Ichazo and Claudio Naranjo (where the symbol was used in a very different way from how Gurdjieff used it).