Aniston's ex Justin Theroux made the announcement via Instagram on Monday and Cox went straight to the comment section to write, "We love you Dolly [two broken heart emojis]."
Cox wasn't the only celebrity pal to reach out. Others like Orlando Bloom and Olivia Munn commented on the post as well, with Munn using nine red heart emojis and Bloom writing "rip beauty."
Theroux shared a slide show of sweet photos on Instagram to commemorate the white German Shepard's life.
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Tonight, at sunset, after a heroic struggle... our most loyal family member and protector, Dolly A. laid down her sword and shield. 🐺💔 She was surrounded by her entire family. “The one that never deserts, the one that never proves ungrateful... is the dog— faithful and true, even in death.” -George Vest Rest In Peace Dolly #adoptdontshop #rescuedog
The sweet pup was covered in petals and wrapped in a blanket. In one of the pictures Theroux is hand-in-hand with someone who fans believe to be Aniston herself.
The actress adopted Dolly back in 2006 and says that she and her companion have a lot in common.
"I'm like Dolly," she told Allure in 2011. "Her brain moves faster than her body, and she's slightly klutzy, which I tend to be."
She also recently shared with fans on Jimmy Kimmel Live! who their four-legged friend was named after.
"I guess I slightly offended her," the 50-year-old joked when she told Kimmel she named her after country music legend Dolly Parton. "I just think it's cute. She's so sweet."
Not long after Aniston's 2018 interview, Parton commented on it as well admitting that she was flattered the actress would name her dog after her.
"It was embarrassing, kinda," Parton said after she mentioned she had the pleasure of meeting Dolly. "Because there were several people there, and I'd hear [Jennifer] say, 'Dolly, don't pee on the couch! Don't be knocking the water bowl over. But actually, I think, probably, the fact that Jennifer has a little dog named after me is cute."
Aniston and Theroux were together for two-and-a-half years — in which they included Dolly and their other dog, a pit bull-boxer mix named Sophie, in the wedding — before calling it quits in February 2018.