In the US Code, it is Title 18, Part 1, Chapter 3, ¶47.
The name came from the woman who spearheaded the legislation, Velma B. Johnston, later known as "Wild Horse Annie".
The Act (since slightly amended but still "on the books") was as follows:
"Whoever uses an aircraft or a motor vehicle to hunt, for the purpose of capturing or killing, any wild unbranded horse, mare, colt, or burro running at large on any of the public land or ranges shall be fined under this title, or imprisoned not more than six months, or both.
"Whoever pollutes or causes the pollution of any watering hole on any of the public land or ranges for the purpose of trapping, killing, wounding, or maiming any of the animals referred to in subsection (above) shall be fined under this title, or imprisoned not more than six months, or both."
References:
? wildhorsepreservation.com
? ispmb.org/annie.shtml