“People eat meat and think they will become as strong as an ox, forgetting that the ox eats grass” (“La gente mangia carne e pensa: ‘Diventerò forte come un bue’. Dimenticando che il bue mangia erba”) is a saying that has been printed on many images (also in the Italian version). The saying was written by Italian actor and author Pino Caruso (1934-2019) in his book Il diluvio universale (1993).
   
The vegan saying has been popular in English since at least 2010.
 
         
Wikiquote: Pino Caruso
Giuseppe “Pino” Caruso (12 October 1934 – 7 March 2019) was an Italian actor, author and TV-personality.
 
Quotes
La gente mangia carne e pensa: “Diventerò forte come un bue”. Dimenticando che il bue mangia erba.
Mangiarsi con gusto un animale è assassinio premeditato a scopo di libidine. Digerirlo, occultamento di cadavere.
. People eat meat and think they will become strong as an ox, forgetting that the ox eats grass.
  Eating an animal with gusto is premeditated lust murder, and digesting it is concealment of corpse.
. Il diluvio universale: acqua passata (Palermo: Novecento, 1993), p. 179.
   
 
Google Books
Il diluvio universale:
acqua passata : parliamoci, finché siamo in vita, dopo non è detto che sia possibile
The universal deluge:
past water: let’s talk to each other, as long as we are alive, afterwards it is not said that it is possible
By Pino Caruso
Palermo, Italy: Novecento
1993
Pg. 179:
In Italia, la vivisezione è vietata. Tranne quando è permessa.
In Italy, vivisection is prohibited. Except when it’s allowed.
 
La gente mangia carne e pensa: “Diventerò forte come un bue”. Dimenticanto che il bue mangia erba.
People eat meat and think, “I will become strong like an ox.” Forgotten that the ox eats grass
 
Mangiarsi con gusto un animale è assassinio premeditato a scopo di libidine.
Tastefully eating an animal is premeditated murder for the purpose of lust.
 
Google Groups: free.it.barzellette
Vegetariani (2)
Dr. Zap
8/28/02
La gente mangia carne e pensa che diventerà forte come un bue.
Dimenticando che il bue mangia l´erba. (Pino Caruso)
People eat meat and think it will become as strong as an ox.
Forgetting that the ox eats grass. (Pino Caruso)
 
Google Books 
Humanimal
By Vergil Z. Ozeca
Lulu Press (Lulu.com)
2010
Pg. 184:
People eat meat and think they will become as strong as an ox. Forgetting that the ox eats grass.
Pino Caruso, Italian actor
 
Twitter
SHI▽I△N × ☉R△CLE
@Shivian
“People eat meat and think they will become as strong as an ox, forgetting that the ox eats grass.” - Pino… http://tumblr.com/xqefi0pzt
10:34 AM · Aug 11, 2010·Tumblr
   
Google Books 
Vegetarian to Vegan:
Give Up Dairy, Give Up Eggs for Good
By Sarah Taylor
Gig Harbor, WA: The Vegan Next Door
2013
Pg. ?:
“People eat meat and think they will become as strong as an ox, forgetting that the ox eats grass.” – Pino Caruso
 
Twitter
♡ bambi ♡
@bbambz
“people eat meat and think they will become as strong as an ox, forgetting the ox eats grass to be strong”
2:14 PM · May 6, 2016·Twitter for iPhone
 
Google Books 
The Skeptical Vegan:
My Journey from Notorious Meat Eater to Tofu-Munching Vegan—A Survival Guide
By Eric C. Lindstrom
New York, NY: Skyhorse Publishing
2017
Pg. ?:
People eat meat and think they will become strong as an ox, forgetting that the ox eats grass. —Pino Caruso
 
Twitter
FAB #Veggie #Vegan
@fabveggievegan
People eat meat and think they will become strong like an ox, forgetting the ox eats grass. #quote #vegan
1:30 AM · Feb 24, 2020·The Social Jukebox