It needs to be stated that there are two very distinct views and teachings within the Buddhist-sphere that being the Theravada and Mahayana.
Now, Theravada is old and originates in Southern Asia (Burma, Thailand and parts of India), this however is not to say that it is the original Buddhism, for Theravada where the Sarvastivada, that is a school of “the theory of all exists”.
Mahayana is also old, and the roots take hold in Tibet, Northern India, China and the Far East. Much of what Mahayana is today rides on-top of the local religions of the people, such as Tibetan Bon, in where the Mahayana teachings are presented through those local religions, however, there are pure Mahayana text that delve deep in to the metaphysics.
Now, Theravada does not recognize Mahayana as legitimate. They do not recognize the Bodhisattva nor even have a concept for the term bodhicitta because that concept simply does not appear in any Theravada sutta.
Theravada and Mahayana have similarities, but that is about it. It is liken the differences between Judaism and Christianity in where there are some common similarities, yet both are distinct and separate teachings and paths. Islam is often called the younger sibling of Judaism in where there are similarities and a lot of crossing of paths, yet there is distinct distinctions that truly make Islam separate from Judaism.
The Mahayana is so radically different than Theravada, it begs to be deemed its own branch within the larger Buddhist-sphere as how Catholic is a sect with in Christianity, that is distinctly different than Baptist.
For many modern Buddhism students, there is a great confusion between Theravada and Mahayana, in where many students end up reading Theravada suttas and commentaries believing that it is Mahayana. It does not help when the Theravadans claim they are the original Buddhism when they clearly are not! Furthermore, the Theravadans further claim that the Mahayana is heresy and dissuade the study or learning of the Mahayana text.
Mahayana on the other-hand, deems Theravada as Hinayana or the “Lesser Path’, for those who are dull or unable to grasp the Mahayana. This view tends then to make Mahayana and elitist religion, in where one has to have the mental facilities to comprehend the Mahayana teachings.
Zen students have to understand and be able to differentiate between Theravada and Mahayana else they will be consumed by confusion and contradicting teachings!