The Skeletroopers are armed skeleton creatures said to exist in the Verdant Meadows Airstrip.
The creatures in San Andreas are based on the characters of the same name from MapleStory, an online role-playing game that was released in 2003. It can be taken as a possibility for the addition of these creatures since GTA San Andreas was released in 2004, a year after MapleStory. However, it is more likely that Rockstar didn't include it due to copyright violations.
Skeletroopers technically do not exist in the game and are a direct result of the famous GTA mod, Mythemix. The Skeletroopers in mod wear black suits with red textures, as compared to the traditional battle armor seen in MapleStory.