The passage you are probably referring is in Act 1, Scene 1, where Iago and Roderigo are arousing Brabantio to inform him that his daughter Desdemona has eloped with Othello.
Zounds, sir, you're robbed! For shame, put on your gown!
Your heart is burst; you have lost half your soul.
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.
Arise, I say!
It is not Iago but Othello who is compared to a devil. Evidently it was common in Elizabethan times to depict the devil in pictures as being black. Othello has to endure a lot of insults throughout the play because of his race and his dark complexion, which are in sharp contrast to his chosen bride whose complexion he describes in Act 5, Scene 2, when he is about to kill her.
Yet I'll not shed her blood,
Nor scar that whiter skin of hers than snow,
And smooth as monumental alablaster
Alablaster, or alabaster, is white marble. Othello probably loved Desdemona as much for her lily-white complexion as for any of her other qualities.
Emilia calls Othello a devil, among other names, in the last act when she finds he has killed Desdemona:
She's like a liar gone to burning hell!
'Twas I that killed her.
O, the more angel she,
And you the blacker devil!
She turned to folly, and she was a whore.
Thou dost belie her, and thou art a devil.
Iago manages to be well thought of by everybody until his villainy is exposed.
Iago compares Othello with black lamb or soever. In first act there is dialogue between Iago and Roderigo.