People often worry about health issues or having less time together. Senior dogs<\/a> have some great advantages, though, that shouldn\u2019t be overlooked. They\u2019re usually already house-trained, calmer, and possess a deep, almost instant gratitude for a new place to rest their heads.<\/p>\n\n\n\n Muttville Senior Dog Rescue<\/a> in San Francisco specializes in giving these older dogs<\/a> a second chance. But preparing an older dog<\/a> for a new home requires a very specific kind of vetting. Specifically: Can this dog coexist with a cat?<\/em><\/p>\n\n\n\n There is a final test that Muttville\u2019s rescue dogs must pass before they can be placed in a new home. And for years, that test\u2019s name was Mustache.<\/p>\n\n\n\n Now, Mustache wasn\u2019t a dog trainer\u2014he was a cat. The ultimate, unflappable feline tester. Mustache\u2019s composed nature made him the perfect choice to determine whether a rescue dog was anxious, aggressive, or totally fine around cats. By just sitting there, unbothered and orange-furred, Mustache helped pave the way for countless senior dogs to find their furever<\/em> homes. On Instagram<\/a>, Muttville stated, \u201cMustache was so confident and so calm\u2026 he was ideal for cat testing.\u201d<\/p>\n\n\n\n There was only one complication: Mustache was a senior himself. After years of dedicated service, the calm, cool, and collected feline tester passed away from advanced kidney disease. In his wake, he left behind a cat-sized hole in the hearts of the Muttville staff. Mustache also left a critical job vacancy. Who could possibly inherit the furry mantle of the ultimate cat tester? And why is cat compatibility so important? <\/p>\n\n\n\n![\"senior](\"https:\/\/www.upworthy.com\/wp-content\/uploads\/sites\/2\/2026\/06\/43_Old_dog_-_senior_labrador_dog_with_collar_resting_on_sidewalk.jpg\")
<\/figure>\n\n\n\nMuttville\u2019s secret weapon<\/h2>\n\n\n\n
The science of furry friendships (and a bittersweet goodbye)<\/h2>\n\n\n\n