What do we know about Apollin, one of the Islamic images in the Poem of El Cid? He is mentioned in chapters 187 and 192.
The only copy of Cantar del mio Cid I have has only 152 chapters in three cantos, and although there are two Moors mentioned whose names begin with "A" I found no mention of an Apollin or any "Islamic image." However, there is an Apollin in another medieval poem, le Chanson de Roland, or the Song of Roland. Dating from perhaps the middle of the 11th century, it mythologizes a relatively minor battle in the Pyrenees between Christian and Muslim forces. In that poem the Saracens are depicted as all being wealthy and caring nothing for God, having a temporal ruler named Muhamet and worshiping a variety of gods, the chief of whom is named Apollin.
The concept that the "infidels" worship many gods instead of one (Allah) may have had its origins in the fact that the Sabeans (the ethnic group of the Prophet Mohammed) before Islam worshiped a wide pantheon of gods and goddesses, of which Allah, the moon god, was chief. There may have been remnants of earlier beliefs among Semitic peoples living in the Iberian peninsula at the time of the writing of the earlier poem.
There are, of course, other versions of the Ed Cid story, one from a 14th century copy of a 12th century text, a 17th century French play by Pierre Corneille and an early Latin work from the 11th century, all of which have many differences including number of chapters and a few minor differences in characters. The links below include the eNotes reference work, a history of the story in general and a full translation of Cantar del mio Cid.