Kidaris

Kidaris (Ancient Greek: κίδαρις) or kitaris (Ancient Greek: κίταρις) was a head ornament of Persian clothing worn by monarchs of ancient Persia.

In many English language works, this is often translated as "tiara" -- using the word with which some contemporary non-Persian writers described it -- but it was not a mundane garment, but a special, ceremonial item that by law was to be worn only by the king. To don the kidaris was to declare oneself king, as many usurpers, such as Baryaxes, were known to have done.

It appears it may have been specifically part of a ceremonial outfit for horseback riding, rather than court attire.