Fascinating update on the Ḏū l-Qarynayn ("the Horned Man") = Alexander the Great front thanks to an amazing discovery reported by C.A. Stewart in “A Byzantine Image of Alexander: Literature in Stone,” Report of the Department of Antiquities Cyprus 2017 (Nicosia 2018): 1-45 ...
The excavations overseen by Dr. Eleni Procopiou at Katalymata ton Plakoton, an Early Byzantine site within the Akrotiri Peninula on Cyprus uncovered a 7th cent. martyrium containing what's being called the "Alexander-Heraclius Stele." It's significant for all sorts of reasons ... 
(e.g., it provide contemporary evidence for the prominence of Alexander in the propaganda of Heraclius), but let's just take a quick look at the iconography of Alexander. Even its damaged state, horns are visible (left); cf. Stewart's careful reconstruction (right) ...
The take-away is obvious (though it should have been obvious already): the identification of the qurʾanic Ḏū l-Qarnayn with Alexander is supported by well-attested iconography of Alexander with “two-horns”, a contemporary literary text ...
with numerous textual parallels (i.e., the Syriac Alexander legend), the early literary reception of the Alexander legends in Arabic which makes this identification, and NOW seventh-century Byzantine iconography of Alexander with two horns that is contemporary with the Qurʾan.
