Monkey Brian is an African monkey that looks like Brian Griffin, though he has no known connection.
In "Untitled Griffin Family History", Nate Griffin roamed the forests of Africa, where he met up with Monkey Brian. Wisely, Monkey Brian warned Nate about how white people had been spotted in those woods and they were kidnapping and enslaving people like him. Foolishly, Nate ignored this warning, only to shortly afterward, get caught by Cleveland from South Carolina.
When Nate escaped from slavery and started up his own family with Lois Laura Bush Lynne Cheney Pewterschmidt, he managed to track down Monkey Brian, kidnap him, and keep him as a pet, against his own will. Monkey Brian commented that technically, this was no different than slavery, but Slave Stewie just tossed him a banana and he chased after it, going "Ooh ooh ah ah! Ooh ooh ah ah! Ooh ooh ah ah!" Nate also chased after the banana, making ape noises as well, making a racist joke about how black people are monkeys.