1891 Slack Post Office was established on December 28, 1891. Slack was in Sheridan County, a few miles west of Parkman near Pass Creek. A newspaper column first titled "Pass Creek Pointers" then "Slack Pointers" appeared irregularly in the Sheridan Post from 1890 to 1892.
1917 Cactus Post Office in Campbell County was established on December 28, 1917. It was named for the cactus on the surrounding plains. A proposed name for the post office was "Shultz" after the postmaster, John W. Shultz. The post office was discontinued in 1933, its duties assumed by the Pine Tree Post Office. Pine Tree is still listed on topographic maps of Wyoming, but Cactus is not.