Most likely the hornet is a metaphor for the terror or panic that an encounter with the Lord would engender for his enemies. This is the way it appears to be used in Exodus 23:27–28. Some alternative suggestions are that it refers to actual hornets that were sent by the Lord to terrorize the armies of Canaan or the Pharaoh of Egypt since their army symbol was a bee.1
12 And I sent the hornet before you, which drove them out before you, the two kings of the Amorites; it was not by your sword or by your bow.