both originated in China and the difference between Zen Buddhism and other forms is the likely result of its mixing with Taosim
Whilst Toaism originated in China, Ch'an/ Zen Buddhism didn't originate there, but developed there. It was an import and adapted to the cultural conditions - for example there's no alms round in China or Japan due to different attitudes to work ethics. Zen/Ch'an displays this superficial difference, but is scripturally and in practice, it is consistent with other forms of Buddhism. I would dispute that it mixed with Taoism, but there is some kind of relationship, and perhaps conclusions reached by practitioners has similar results. A more modern example of this interchange can be seen with certain Christian Carmelite Monasteries where they have adopted Buddhisy meditation techniques. You couldn't claim from this that the traditions mixed though. As I indicated earlier, my friend suggested that there was a relationship, and probably quite a powerful one.
the true self of an individual beyond identification with phenomena, the essence of an individual." The use of the word "self" is certainly non-Buddhistic, but the way that self is described as being "beyond identification with phenomena
They are not the same - you could probably equate the essence of Hinduism with ideas of a soul, though in Christian traditions this doesn't seem to be a very well developed idea in that there's no attempt to explain what this soul my be or consist of - certainly in the common conception. I think you have to be careful of making assumptions based upon attempts to describe the ideas. Hinduism has this definitive, though difficult to realise, underlying reality whereas the ultimate nature of reality in Buddhism is emptiness, which is not an essence or realm, but a realisation that can be achieved.
I see much commonality between the yogi striving to attain union with God or Atman and the Zen meditator trying to awaken in him or herself what is referred to as the "Buddha-nature."
The difference between Atman and Buddha-Nature is that the Atman is some kind of underlying nature present in the being, whilst Buddha-Nature is a potential and not a thing. All beings have Buddha-Nature, which refers to the possibility that in time they could become Buddhas. This is one of the features of Mahayana Buddhism - of which Zen is a part - and forms the basis of the vows to help all being achieve enlightenment.
http://buddhism.about.com/od/mahayan...dha-Nature.htm
This section in the link has a good description of what I'm getting at.
Is Buddha Nature a Self?
it's a fascinating subject. There are various interpretations, as you can see from the link - the closest to your idea being a subtle mind.