hi
the earlest known refrences to unicorns in classical lititure is by herodotus (484 - 425 BC) who mentions the existence of an indian ’ wild ass’ with one horn, probably meaning the indian rhino.
this was just a passing comment though. the first real discription that sparked a thousand rumours was by the greek historian ctesias of cnidas in the fourth century BC. he had been a doctor to derius two and artaxerxes two, emperors of, persia, india and the far east which he wrote down in his book persica whe he retired back to greece.
this book inspired the young alexander the great to want to see that places for himslf; which he famously did through conquring half of them.
ctesias had no first hand expirence of unicorns but recorded what he heard from people he believed reliable.he also probably handed a coloured drinking cup made from indian rhinoceros horn which he was told that came from a unicorn.