A messenger sent before to give notice of the approach of others; a harbinger; a sign foreshowing something; a prognostic; as, the forerunner of a fever
If you describe a person or thing as the forerunner of someone or something similar, you mean they existed before them and either influenced their development or were a sign of what was going to happen. a machine which, in some respects, was the forerunner of the modern helicopter = precursor