This is one of the 23 delegations (satrapies) shown bringing offerings to the King during the New Year (Nou Rutz) festival.
The bas relief of each delegation provides valuable insight into the clothes and gifts from each region at that time.

The "Ethiopians" are shown 1st on the left at the beginning of the oblique (slanting) tier of the Apadana stairway.

They are represented by 3 members, and bring a "mini-giraffe", an "elephant tusk" and a covered vessel.

There is some debate about whether these delegates are from Ethiopia as we know it, bearing in mind that Ethiopia was not part of the Persian empire. The term Ethiopian means “burnt face” in ancient Greek and could apply to anyone of dark complexion. Furthermore, given the accuracy with which all other animals have been portrayed it seems likely that the "mini-giraffe" is more likely some other form of antelope.
They have otherwise been identified as Maka or Nubians.

The original bas relief is shown in one of the photographs (with low tonal range due to the shade).
The other photographs have been digitally enhanced by blurring the background in order to reveal their otherwise hidden beauty. I hope you find them as fascinating as I do.