In the first act of Othello, Iago is upset that Othello and Desdemona have gotten married. He uses racist and inflammatory language to incite Brabantio to rage, apparently hoping he can manipulate Brabantio into violence against Othello.

He alerts Brabantio with great alarm, implying something terrible has happened, crying "Arise, arise!" as if Othello has kidnapped his daughter and raped her. He tries to raise animalistic images in Brabantio's mind of Othello having sex with his daughter, calling Othello an old black ram and contrasting his blackness with Desdemona's whiteness as a "ewe." Iago is using this imagery to try to convey that the marriage is unnatural.

He also refers to Othello as the devil, leaning into an idea at the time that the devil was black:

Even now, now, very now, an old black ram
Is tupping your white ewe. Arise, arise!
Awake the snorting citizens with the bell,
Or else the devil will make a grandsire of you

He refers to Othello as a Barbary horse having sex with his daughter. (A Barbary house was an African horse.) This again insinuates that Othello is racially "other" and an animal.

Iago goes onto imply that Othello's relatives are animals, nephews who neigh like horses, for example:

You'll have your daughter covered with a Barbary horse,
you'll have your nephews neigh to you

Iago also uses the word "Moor," which means a black person from North Africa, to refer to Othello, saying, he, Iago, has come:

to tell you your daughter
and the Moor are now making the beast with
two backs

Iago seems obsessed with the idea that Othello is having sex with a white woman and tries as hard as possible to communicate to Brabantio that this is sordid, unnatural, and disgusting. He is successful in inciting Brabantio, but fortunately, Desdemona is able to reassure her father quite earnestly that she married Othello willingly and is in love with him.