The MLB was in PETA's sights in 2021. In case you missed it, PETA, the animal rights nonprofit organization, called out Major League Baseball for referring to the warm-up area for pitchers as the "bullpen."
According to PETA, cows and bulls are frequently housed in bullpens before getting slaughtered for the meat industry. Thus, in PETA's opinion, the term "bullpen" makes fun of the animals' unheard suffering.
Tracy Reiman, the Executive Vice President of PETA said:

"Words matter, and baseball 'bullpens' devalue talented players and mock the misery of sensitive animals. PETA encourages Major League Baseball coaches, announcers, players, and fans to changeup their language and embrace the 'arm barn' instead."
In October 2021, the People for the Ethical Treatment of Animals organization requested Major League Baseball to change the name to "arm barn."
""Bullpen” refers to the area of a “bull’s pen” where bulls are held before they are slaughtered—it’s a word with speciesist roots & we can do better than that. Switching to “arm barn” would be a home run for baseball fans, players, and animals." - @PETA
MLB fans didn't miss the opportunity to make fun of PETA's call.
"Only way I’m calling it “arm barn” is if it looks like this next year." - Dave
Even the MLB Tampa Bay Rays joined in the meme fest.
"Tired: Arm Barn; Wired: Ray Tank." - Tampa Bay Rays
How did the warm-up area for MLB pitchers come to be known as the "bullpen"?
"Bullpen" has been used in MLB for a century

According to an ESPN article, Paul Dickson explains the origin of the term in his book "The New Dickson Baseball Dictionary."
Per Paul, the term was first used to refer to the area where baseball pitchers warm up in the December 1915 issue of Baseball Magazine. Since the outfield wall of almost every baseball stadium had a giant bull-shaped billboard that served as the warm-up area for relievers, the area eventually earned the nickname "bullpen."