@Charied1 The creature could be a genetically engineered werewolf. The goal was to create an army, but they discover the plan and destroy the entire hidden laboratory. They manage to save, or believe they save, a young boy, whom they then take to an orphanage. Rosalie and Emmett later adopt him, and over time, the whole family discovers what he is and helps him. As a genetically modified werewolf, he possesses consciousness when he transforms and can transform at will without needing a full moon. He also has rapid regeneration, making him immortal, and he grows to a certain age before ceasing to age.
It's just an idea; I don't know if it helps you at all.