He could be a prophet of Allah or not.
"And for (almost) every nation (or community), there is a Messenger; when their Messenger comes, the matter will be judged between them with justice, and they will not be wronged." (Qur'ân 10:47)
"And verily, We have sent among every Ummah (community, nation) a Messenger (proclaiming): "Worship Allah (Alone), and avoid (or keep away from) Taghut (all false deities, etc. i.e., do not worship Taghut besides Allah)." Then of them were some whom Allah guided and of them were some upon whom the straying was justified. So travel through the land and see what was the end of those who denied (the truth)." (Qur'ân 16:36)
". . . And there never was a people, without a warner having lived among them (in the past)." [Qur'an 35:24)
"of some messengers we have already told thee the story; of others we have not." (Qur'an 4:164)
Narrated jabir bin abdullah: The Prophet said, "every prophet used to be sent to his nation only but i have been sent to all mankind."
Abu Dharr said: I said: O Messenger of Allah, how many Prophets were there? He said: “One hundred and twenty four thousand.” I said: O Messenger of Allah, how many of them were Messengers? He said: “Three hundred and thirteen, a good number.” I said: O Messenger of Allah, who was the first of them? He said: “Adam.” …
So was Zarasutra, as well as Khrisna, Zoroaster, Sirus, Sidharta prophets of Allah? wallahu allam.