This line is from Act IV, Scene I. In this scene, some time after the death of Caesar, the Republic is in turmoil. With Antony, Lepidus, and Octavius in charge--the triumvirate--Antony and Lepidus compile a list of their political enemies and mark ("prick'd") the names of those who must die. Octavius tells Lepidus that his brother must die; Lepidus agrees on the condition that Antony's nephew, Publius, die. Marc Antony agrees. Then, when Lepidus leaves, Antony tells Octavius of his plans to use Lepidus for his political objectives and then cut him off:
Octavius, I have seen more days than you;/And though we lay these honors on this man,/To ease ourselves of divers sland'rous loads [various burdens of blame]/To groan and sweat under the business, Either led or driven, as we point the way;/And having brought our treasure where we will,/Then take we down his load, and turn him off,/(Lide to the empty ass) toshake his ears/And graze in commons [public pastures] (IV,i,18-27)
When Octavius objects, saying that Lepidus is a "tried and valiant soldier" (IV,i,28), Antony coldly retorts, "So is my horse...." (IV,i,29). This remark is much different in tone from Antony's loving words said over Caesar's body in Act III, scene 1.
That power corrupts is evidenced in this scene as the triumvirate move men's lives around as those they are pieces on a chess board. Ironically, the corrupt political power of these men well superpasses the tyranny which Brutus feared from Caesar. And, also ironically, Antony's prediction in Act III that
A curse shall light upon the limbs of men;/Domestic fury and fierce civil strife/Shall cumber all the parts of Italy (III,i,263-265)
is fulfilled by Marc Antony himself!