When the Bell AH-1 Cobra gunship was being developed, the Army's intent was to name the aircraft after an American Indian tribe as had been its long tradition. However, at the time of the development of the Cobra gunship, the Department of Defense was in a legal battle with Piper Aircraft who also named their aircraft after American Indian tribes.
In Vietnam, UH-1 "Hueys" were called "Slicks" if they were operating unarmed, "Hogs" if they were armed with rockets, and "Cobras" if they were armed with guns. As Bell's main pitch for the procurement for the AH-1 was that it was a low-risk development of the UH-1 Iroquois (to some degree true given that the original gunship proposal was called the Iroquois Warrior and had more in common with the UH-1 than the final AH-1 design), the AH-1 got named "Huey Cobra".
Source: New Vanguard: Huey Cobra Gunships by Chris Bishop. Osprey Publishing, 2006, p6-7.