An elongated human face that most closely resembles either the face of an animal from the Equidae family, of the face of an adult that most probably suffers from Fetal Alcohol Syndrome. The most prominent and public example of a horseface is the one recently displayed by Eli Manning after the Philadelphia Eagles defense destroyed his will.
Holy crap, did you see Eli's horseface after he threw an interception to Asante Samuel? It is so nasty that I just threw-up in my mouth. He really looks scared out there.