The phrase "brother dog" refers to a close relationship between people, where one person considers another person to be like a brother to them. It compares this bond to the friendship and loyalty one typically experiences with a dog. Full definition
Prince found a wonderful family to love him including a little brother dog just waiting to play with him. (dogs4disabledveterans.org)
Onyx plays with her foster brother dog but is most likely not good with cats. (gsgsrescue.org)
Firstly I do smile sometimes honest... Passion for attending gigs, watching movies, going for walks in the country or around cities, and playing with my dogs (well my brothers dogs). (heavymetaldate.com)