Also known as Zoroaster. The word Zarathustra comes from the Avestan language, Zaraϑuštra, composed of zarat which means ""golden," and uštra which means "star, light." The word became Zoroaster due to how the name was rendered in Greek: Zōroastrēs (Ζωροάστρης).
Zarathustra is a title, just like the titles Buddha ("awakened"), Jesus (Yeshua, "savior"), Manu, Vyasa, Kabir, Rinpoche, Dalai Lama, etc. Unfortunately, just as in other cases, humanity confused the title with a name, and assumed it referred only to one person. Zarathustra is popularly considered to be a Persian man who founded Zoroastrianism and the author of Yasna Haptanghaiti and the Gathas (hymns) of Zoroastrianism. However, the period of time in which he lived is unknown, with suggestions from various writers ranging from the 18th to the 6th centuries BC (a period of 1200 years!). Moreover, as an aside H. P. Blavatsky explained,
"By 'original' we mean the "Amshaspend," called "Zarathustra, the lord and ruler of the Vara made by Yima in that land." There were several Zarathustra or Zertusts, the Dabistan alone enumerating thirteen; but these were all the reincarnations of the first one. The last Zoroaster was the founder of the Fire temple of Azareksh and the writer of the works on the primeval sacred Magian religion destroyed by Alexander." This may explain the difficulties in trying to find a single date to assign to the man." - The Secret Doctrine, 1888
Like the tradition of the Dalai Lamas, the Zarathustras were not all one person, but incarnations of a force, an energy. In The Story of Tibet, the Fourteenth Dalai Lama said, "If someone asks me whether I am the reincarnation of the Dalai Lama," he said, "Then I answer, without hesitation, yes. This does not mean that I am the same being as the previous Dalai Lama. Some Dalai Lamas are a manifestation of Manjushri. Some are a manifestation of Chenrezig... I have a special connection with the Thirteenth Dalai Lama and the Fifth Dalai Lama. I have felt some kind of karmic relations or connections even with the Buddha... we might have been born in the same time during a past life; a teacher and a student; or a ruler and a subject or we might even have been spiritual friends." Thus, we find the same scenario in every religion, such as in the case of the Zarathustras.