A 7-year-old Chihuahua cuddling up to his 18-year-old dog brother has touched many hearts online for the sad truth – the older one doesn't have much longer to live, and his best friend knows it.

A viral TikTok video captured the heartbreaking moment when Toby the Chihuahua realized that his brother Huang'er was nearing his final chapter in life.

"Huang'er has been receiving treatment for congestive heart failure for the past year," pet owner Hannah Moosvi told Newsweek.

"The condition is really taking its toll. Huang'er can't walk much and needs more daily assistance," she explained.

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In the clip, Huang'er can be seen lying on the floor breathing heavily while Toby, apparently interpreting the moment correctly, cuddles up to him comfortingly.

While the little Chihuahua is still fit and agile, his big brother needs much more rest due to age and illness.

Hannah described the moment as "bittersweet," saying, "I am sad knowing Huang'er is getting older but it's also sweet to see them so bonded. I'm glad Huang'er has someone to watch over him."

Heart failure in dogs occurs when the heart is no longer able to pump blood efficiently, leading to fluid build-up in the lungs, chest cavity, or abdomen.

"Toby is almost 10 years younger and I believe Toby kept Huang'er 'young' for a while, too. Now, Toby has become quite the caretaker. He alerts us whenever Huang'er has an issue," Hannah said.

"They really do everything together."

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The touching video resonated with many viewers who shared their own bittersweet experiences of caring for older pets.

"Huang'er has been part of my life for almost two decades," Hannah said.

"He's gone through so many changes with me, but, on the inside, has always stayed the sweet little puppy who loves everyone. He is still part of everything we do," the pet owner added.