What cultural prejudice in Brabantio is lago referring to when he uses this imagery?

I guess that you are referring to when Iago and Roderigo are at Brabantio's window yelling about his daughter having sex with Othello. They use some standard stereotypical racial slurs for moors or blacks at the time. Much of it has to do with horse imagery and certain sexual prowess. Here are some quotes. You'll get the point,

"Even now, now, very now, an old black ram

Is tupping your white ewe."

"Because we come to do you service and you think we are ruffians, you’ll have your daughter covered with a Barbary horse. You’ll have your nephews neigh to you."