There is a collection of Plutarch’s essays known as the Moralia. I found this quote on the Armchair Enlightened Despot:

You ask me why Pythagoras abstained from eating the flesh of beasts, but I ask you, what courage must have been needed by the first man who raised to his lips the flesh of the slain, who broke with his teeth the bones of a dying beast, who had dead bodies, corpses, placed before him and swallowed down limbs which a few moments ago were bleating, bellowing, walking, and seeing? How could his hand plunge the knife into the heart of a sentient creature, how could his eyes look on murder, how could he behold a poor helpless animal bled to death, scorched, and dismembered? how can he bear the sight of this quivering flesh? does not the very smell of it turn his stomach? is he not repelled, disgusted, horror-struck, when he has to handle the blood from these wounds, and to cleanse his fingers from the dark and viscous bloodstains?

The scorched skins wriggled upon the ground,
The shrinking flesh bellowed upon the spit.
Man cannot eat them without a shudder;
He seems to hear their cries within his breast.

Thus must he have felt the first time he did despite to nature and made this horrible meal; the first time he hungered for the living creature, and desired to feed upon the beast which was still grazing; when he bade them slay, dismember, and cut up the sheep which licked his hands. It is those who began these cruel feasts, not those who abandon them, who should cause surprise, and there were excuses for those primitive men, excuses which we have not, and the absence of such excuses multiplies our barbarity a hundredfold.

‘Mortals, beloved of the gods,’ says this primitive man, ‘compare our times with yours; see how happy you are, and how wretched were we. The earth, newly formed, the air heavy with moisture, were not yet subjected to the rule of the seasons. Three-fourths of the surface of the globe was flooded by the ever-shifting channels of rivers uncertain of their course, and covered with pools, lakes, and bottomless morasses. The remaining quarter was covered with woods and barren forests. The earth yielded no good fruit, we had no instruments of tillage, we did not even know the use of them, and the time of harvest never came for those who had sown nothing. Thus hunger was always in our midst. In winter, mosses and the bark of trees were our common food. A few green roots of dogs-bit or heather were a feast, and when men found beech-mast, nuts, or acorns, they danced for joy round the beech or oak, to the sound of some rude song, while they called the earth their mother and their nurse. This was their only festival, their only sport; all the rest of man’s life was spent in sorrow, pain, and hunger.

‘At length, when the bare and naked earth no longer offered us any food, we were compelled in self-defence to outrage nature, and to feed upon our companions in distress, rather than perish with them. But you, oh, cruel men! who forces you to shed blood? Behold the wealth of good things about you, the fruits yielded by the earth, the wealth of field and vineyard; the animals give their milk for your drink and their fleece for your clothing. What more do you ask? What madness compels you to commit such murders, when you have already more than you can eat or drink? Why do you slander our mother earth, and accuse her of denying you food? Why do you sin against Ceres, the inventor of the sacred laws, and against the gracious Bacchus, the comforter of man, as if their lavish gifts were not enough to preserve mankind? Have you the heart to mingle their sweet fruits with the bones upon your table, to eat with the milk the blood of the beasts which gave it? The lions and panthers, wild beasts as you call them, are driven to follow their natural instinct, and they kill other beasts that they may live. But, a hundredfold fiercer than they, you fight against your instincts without cause, and abandon yourselves to the most cruel pleasures. The animals you eat are not those who devour others; you do not eat the carnivorous beasts, you take them as your pattern. You only hunger for the sweet and gentle creatures which harm no one, which follow you, serve you, and are devoured by you as the reward of their service.

‘O unnatural murderer! if you persist in the assertion that nature has made you to devour your fellow-creatures, beings of flesh and blood, living and feeling like yourself, stifle if you can that horror with which nature makes you regard these horrible feasts; slay the animals yourself, slay them, I say, with your own hands, without knife or mallet; tear them with your nails like the lion and the bear, take this ox and rend him in pieces, plunge your claws into his hide; eat this lamb while it is yet alive, devour its warm flesh, drink its soul with its blood. You shudder! you dare not feel the living throbbing flesh between your teeth? Ruthless man; you begin by slaying the animal and then you devour it, as if to slay it twice. It is not enough. You turn against the dead flesh, it revolts you, it must be transformed by fire, boiled and roasted, seasoned and disguised with drugs; you must have butchers, cooks, turnspits, men who will rid the murder of its horrors, who will dress the dead bodies so that the taste deceived by these disguises will not reject what is strange to it, and will feast on corpses, the very sight of which would sicken you.’