Henry the dog is featured in the 2020 book Our Wild Tails: The Adventures of Henry & Baloo
On Saturday, she shared three pictures featuring the dog along with a message about how "Henry went really downhill overnight" and was rushed to the emergency vet.
"When we got up he could barely walk, lost so much muscle over just a few days, his balance was off, struggled with breathing normally and wouldn't eat anything even with the steroids he's on" Bennett wrote.
"He had to spent [sic] the night because he could barely lift his head once we got there. He needs radiation on his lymph nodes to help with his breathing and to get him through this," the caption continued. "I don't have words for how we're feeling, we've never experienced pain like this. It's absolutely horrible. Animals don't deserve this, no one deserves this. So many are sending you love Henry, I hope you can feel it."
The pictures showed Henry — a German Shepherd, Husky, Boxer, Staffordshire Terrier and Aussie mix — laying on the ground outside with his best cat friend Baloo reclining against him.
Another image showed a shrine for the canine, while a video saw him laying on the ground with his flank shaved and Baloo cuddled close next to him.
Bennett updated Henry's 2.2 million followers on Monday via Instagram Stories with the sad news that his "cancer is winning."
"This is unbelievably hard to write… I think because it makes our situation that much more real," she wrote at the bottom of a photo of herself with the dog.
"Henry making it through the night and through radiation was a miracle, but the cancer is winning. It's completely through his entire system and our vets said he's not strong enough to keep getting treatment."
"I can't believe I'm saying this, but him making it and not going in a hospital is a miracle and we're so grateful to get to give him a couple more days at home surrounded by love, fresh air and his best friends," she continued.
"I'll share more of our day together later tonight on our stories. But right now we're headed to take him to the mountains one last time. It's a beautiful day and even though he's super weak, we know he wants to go see what he loves most. Please send him lots of hugs and love. He's given so much and deserves every bit of it."
Henry and Baloo — a Lynx Point Siamese and Norwegian forest cat mix — frequently appear on their Instagram account posing together on various outdoor adventures, from frolicking in the snow to posing lakeside.
In mid-February, Bennett first informed followers about Henry's illness, writing alongside an image of the animal pair, "Our sweet Henry has been diagnosed with cancer. It's one of the bad ones. We are heartbroken. For ourselves and for our baby Baloo. We are already crying for the decisions to come. But Henry is the toughest dog around, and there is definitely some fighting to be done."