The closest thing we got are fungus, which form complex network and work together to survive, but such network lack the cells for what we define as intelligent, neurons, if a species of single cell life form manage to develop a neural network without becoming a multicellular organism, I guess it would be possible, but highly unlikely, because they don't need to, just look back at fungus, they are perfect as they are, evolution isn't driving them to becoming a thinking mass of slime to survive, they are doing great as it.