"This mushroom happened to have a lethal toxicity and within hours it was rapidly destroying his liver and immune system to the point of no return," Johnson wrote. "I encourage all of you out there to be mindful of mushrooms in your yards, parks or anywhere outside your dogs play. What looks innocent, can be deadly to your lil' family members."
The 43-year-old said that he "held [Brutus'] lil' paw as he was finally at peace."
"I set them both down and they both take off in a full sprint and fall right into the deep end of our pool," he wrote. "HOBBS immediately starts doggy paddling while BRUTUS (like a brick) sink heads first to the bottom of the pool. I take off into a full sprint, fully clothed, dive in the pool, swim to the bottom, rescue my brick, I mean BRUTUS and bring him back to the edge of the pool. He was a little delirious.. took a moment, threw up all the water he swallowed and looked up at me as if to say, 'Thank God you didn't have to give me mouth to mouth!' and then ran off to play with his brother."
In his post on Tuesday, Johnson seemed to reference the moment in a hashtag that read, "#WishICouldveSavedYouOneMoreTime."