Old Brown is a fictional character from the children's book series The Adventures of Tintin, created by Hergé. He is a large, fierce-looking dog known for his distinctive brown fur and grumpy demeanor. Old Brown is often seen guarding the property of Professor Calculus and is known for his loyalty and protective nature. In the story, Old Brown plays a significant role in the plot, particularly in the adventure titled The Secret of the Unicorn. His interactions with the main characters, including Tintin and Captain Haddock, add humor and tension to the narrative, showcasing the bond between humans and animals.