When did Idaho make poaching a law?

Answer:

Idaho's first game laws were established by the Idaho Territorial Legislature in 1864.  These laws for closed seasons on big game animals from Februrary to July, but did not provide for enforcement. These laws predate both the State of Idaho (admitted to the Union in 1890) and the creation of the Department of Fish and Game in 1899 which provided an enforcement mechanism.  Deputy wardens were hired in each county and were paid half of each fine for which they could get a conviction. 

You can view the Fish and Game History for more background on the Department, or browse IDAPA Code to review the rules governing Fish and Game and game law.