The Haradrim were supposed to have a serpent on their banner. During the Battle of the Pelennor Fields, Theoden felled "the black serpent" on a field of red, the banner of the Haradrim who fought under Sauron.
As for the Easterlings, I don't recall if their banner or main battle standards were ever described in any detail.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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