I was under the impression that uruk-hai were orcs bred with goblin men by Saruman, though Sauron still posessed many uruks after the fall of Isengard. According to some sources I remember, some wild men (perhaps Variags or Dunlendings) offered to be changed by Saruman to gain power and fighting prowess. They were half-orcs, part wild man, part orc.
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"What mortal feels not awe/Nor trembles at our name,
Hearing our fate-appointed power sublime/Fixed by the eternal law.
For old our office, and our fame,"
-Aeschylus, Song of the Furies
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