Toby is a rescue dog who was adopted by Indiana couple Jake and Devin Crouch in 2011. In December 2012, the Crouches welcomed another addition to their family - baby Carter, now 18 months old.
All parents with pets hope that the animals will take to their children, but for Jake and Devin the bond that emerged between baby Carter and Toby exceeded their wildest expectations.
Far from causing any trouble, Toby embraced his role as a 'big brother' from the get-go.
"The first Carter and Toby photos started the day Carter came home from the hospital, when we noticed how protective Toby was over him. Toby stayed close to his new brother and would follow anyone who would hold Carter," Devin told the Daily Mail.
In a funny introduction to the family's website, 'Toby' explains the family set-up, referring to Carter as a 'bald, ugly puppy'.
The joke isn't too far from reality, according to the Crouches, who say that Toby and Carter stick together like real brothers.
Devin, a stay-at-home mum, began charting their daily adventures together and uploading them to image sharing site Instagram.
"We realised that the two of them would always have a special relationship and we thought an Instagram account would be a great way to document their lives together."
The photos show Carter and Toby engaging in a range of cute activities together, from playtime to bathtime - one snap even shows the pair 'reading' together in the study.
The Crouches never imagined that this simple idea for a 'family journal' would find an audience outside of family and friends. But if there's anything the internet loves, it's dog and babies, so it's not really surprising that they can now boast over 7,000 fans of the unlikely duo's antics.