The incident involving the bird, known as Sugar, occurred Tuesday afternoon on Mohawk Street.
"I saw him get hit with the club, and it was horrible," Pasadena resident Gina Davis told Eyewitness News.
Residents described Sugar as a white peacock that is well known in the area and has a reputation for being aggressive at times.
"He could be a little nasty, but he's a peacock. He's a wild animal. He just chases everybody around," Pasadena resident Adela Gaytan said.
Davis said there have been previous instances where Sugar chases the mail carrier but believes the bird did not deserve to be harmed.
"There are times when we've seen him chase the mailman, and I understand, but you can't just throw rocks and hit with sticks," she said. "You have to figure out how to take care of that a little bit better."
Video of the incident appears to show the letter carrier throwing water or another liquid at Sugar before picking up a rock and throwing it at the bird. A short time later, the video shows Sugar standing in front of the mail truck.
The carrier then comes out and hits the peacock with a stick or club.
Sugar flopped around before collapsing. Witnesses said the mail carrier drove away, leaving the bird in the street injured and disoriented.
"He couldn't move his wings. Something was wrong with his eye," Davis said. "Maybe that's where he got hit with the rock. One of his eyes was swollen. It was pretty sad."
The Pasadena Humane Society is investigating the incident and evaluating Sugar for any injuries.
Outraged residents said they hope the letter carrier is held accountable and that Sugar returns to the neighborhood.
"We love him a lot, and we hope he gets to come back to our neighborhood," Davis said.
The U.S. Postal Service said several employees have had run-ins with peacocks in the area, including a recent attack. The agency said it is investigating the incident captured on camera.