【Forerunner】 , 【precursor】 ,  【harbinger】 ,  【herald】  are comparable when they denote someone or something that comes before another person or thing and in some way indicates his or its future appearance.

【Forerunner】 may denote a messenger that goes before a personage (as a king, prince, or lord) to warn others of his approach, but more often the term is applicable to something that serves as a sign, presage, or warning of something to follow.

【Precursor】  commonly carries an implication of making ready or of paving the way for the success or accomplishments of another person or thing rather than, as  【forerunner】 , one of serving as an announcement or prediction of what is to come.

【Harbinger】 occurs chiefly in an extended sense, sometimes applying to a person or thing that announces something which is coming and for which one must be prepared and sometimes applying to one which goes before as a pioneer or initiator.

• Read more: 【Harbinger】 vs Harbringer

【Herald】 basically denotes an official who makes a solemn and stately proclamation or announcement (as of war or peace or the birth of an heir to the throne); in extended use it is applied to something which similarly announces or proclaims.