There earliest proponent of veganism listed on Wikipedia is Abu al-Ala al-Ma'arri, roughly one thousand years ago. According to their source:

"[Al-Maʿarri's] diet was extremely frugal, consisting chiefly of lentils, with figs for sweet; and, very unusually for a Muslim, he was not only a vegetarian, but a vegan who abstained from meat, fish, dairy products, eggs, and honey, because he did not want to kill or hurt animals, or deprive them of their food."

Also:

Al-Ma'arri held an antinatalist outlook, in line with his general pessimism, suggesting that children should not be born to spare them of the pains and suffering of life.

I should read this guy's work.