One of the many was the mysterious Pythagoras.

Fruits and Vegetables - the God given foods
The first prominent modern vegetarian was the brilliant philosopher Pythagoras who lived towards the end of the 6th century BC. The Pythagorean diet came to mean an avoidance of the flesh of slaughtered animals. Pythagorean ethics first became a philosophical morality between 490-430 BC with a desire to create a universal and absolute law including injunctions not to kill "living creatures," to abstain from "harsh-sounding bloodshed," in particular animal sacrifice, and "never to eat meat."

Philosopher and mathematician. He founded a religious brotherhood, which followed a life of strict asceticism and greatly influenced the development of mathematics and its application to music and astronomy.
Quotes (according to Ovid):
from The Extended Circle by Jon Wynne-Tyson
Alas, what wickedness to swallow flesh into our own flesh, to fatten our greedy bodies by cramming in other bodies, to have one living creature fed by the death of another! In the midst of such wealth as earth, the best of mothers, provides, nothing forsooth satisfies you, but to behave like the Cyclopes, inflicting sorry wounds with cruel teeth! You cannot appease the hungry cravings of your wicked, gluttonous stomachs except by destroying some other life. - Depicted in Ovid: The Metamorphoses, translated by Mary M.Innes
As long as man continues to be the ruthless destroyer of lower living beings he will never know health or peace. For as long as men massacre animals, they will kill each other. Indeed, he who sows the seed of murder and pain cannot reap joy and love. - attributed by Ovid
Animals share with us the privilege of having a soul. - attributed by Ovid
If a group of beings from another planet were to land on Earth—beings who considered themselves as superior to you as you feel yourself to be to other animals—would you concede them the rights over you that you assume over other animals?
— George Bernard Shaw
"When you offer a child the choice of a piece of meat or an apple, it's never the meat that he chooses. There's an ancestral instinct there."
- Adolf Hitler