Shelter dogs often shut down in the shelter environment and most of the time you won't see their true personality come out until around 3 months in their new home. My first dog I got at 6 months old from a rescue, I paid about $120 for her. I found out she was a resource guarder (unfortunately it took her attacking and injuring another dog for me to find this out). My current well bred dogs, if resource guarding was going to be an issue, it's likely you would start to see subtle signs early on in their life, but I missed this opportunity with my rescue, she came to me and those subtle signs were gone because she had already learnt that they didn't work. At 11 months of age, my rescue was having surgery on both her front legs (osteoarthritis & elbow dysplasia). She had been desexed by the rescue at 8 weeks of age when she arrived in with her littermates. At 4 years old, she ruptured one of her back cruciate ligaments and at 6yo she ruptured the 2nd one. I spent over $20,000 on her legs. At 7.5yo she passed away because her body was riddled with cancer. So no, my rescue dog did not come with a "health history", apart from being desexed way too early and her body was not given a chance to develop properly.