The incident occurred Tuesday night near the back entrance of the Stein Bar on Washington Boulevard.
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Kayla Maes posted the disturbing footage on the bar's Instagram page.
The video shows the dog peeking out of the entrance when the man suddenly punches him in the face.
"He went flying literally from the front door all the way like maybe 3 or 4 feet to where the start of the bar is and he hit the bar," she said.
Witnesses say the man returned to the bar an hour later but customers kept him there until sheriff's deputies arrived.
"One of my regulars actually tackled him to the ground, he sat on top of him and I called the sheriff's and said, 'He's here again. Hurry, please! Please!"
Deputies with the Pico Rivera Sheriff's Station took the suspect into custody. He was reportedly cited for animal abuse and has since been released.
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The dog named Steven suffered a swollen eye but was given medication and some ice cream to help with the pain.
"Everyone wants to know, 'Where's Steven? Is Steven okay?' There was even a hashtag 'Justice for Steven,'" said Whittier resident Laura Ramirez.
Maes hopes the suspect won't return, but says Steven will soon be back at the Stein to greet his regulars.
"He greets anybody that comes in and there's so many people that message me, 'Oh my God, I was driving by and I saw Steven in the door,' so I feel like everybody in Whittier knows him now."
Eyewitness News reached out to the Pico Rivera Sheriff's Station for more information on the suspect but has not heard back.