What are examples of animal imagery in Othello, and what is their purpose in the drama?

The most significant use of animal imagery is in Act I,
scene i, and it is used in reference to Othello.

Iago and
Roderigo are under Brabantio's window, awakening him to alert him to the fact that his
daughter, Desdemona, has run off with Othello.  Under cover of darkness, Iago
says:

Even
now, very now, an old black ram

Is tupping your white ewe. .
.

And
later:

. .
.you'll have your daughter cover'd with a Barbary horse;
you'll have your nephews neigh to you. . .[Y]our daughter and the Moor are now making
the beast with two
backs.

All of
these derogatory images are meant to anger Brabantio, as Iago and Roderigo refer to
Othello having sex with Desdemona in an animalistic way.  Throughout this scene, it
should be noted, Othello is only ever referred to as the "Moor" or by racial slurs like
"thicklips."  All of this is intended to add up to an expectation of who/what Othello is
when the audience meets him in Act I, scene ii.  How surprised they are to see a
dignified, noble and beautifully well-spoken man arrive onstage, overturning the
expectations created by the opening scene..

So, the animal
imagery aimed at Othello in Act I, scene i is meant to prepare a false perception of him
in the mind of the audience, while also highlighting the prejudiced points of view of
Iago, Roderigo and Brabantio.