[thaumasiotes]

> Xenophobia is literally a "fear of unkown"

It's literally a "fear of foreigners". It was possible to use the adjective ξένος to call something strange, but that's not the sense used in the word, or the most common meaning of ξένος.

Based on the dictionary entries (see https://www.perseus.tufts.edu/hopper/morph?l=cenos&la=greek#... ), it does not appear to have been possible to use ξένος to describe something as unknown. There is a related sense, "ignorant [of ...]", but that would describe the person who isn't familiar with something, not the thing that is unfamiliar to a person.

For "fear of the unknown", you'd presumably want something like "agnostophobia".