Mariposa super mutants are the twisted vision of Richard Grey (better known as The Master) and dominated the pre-NCR Core Region before The Brotherhood of Steel and The Vault Dweller destroyed Mariposa Military Base and The Cathedral. After there defeat most of these mutants scattered and divided into several smaller factions. The first was led by Gammorin, they traveled east inorder to start fresh. The second was lead by Attis who wanted to restart The Masters work and retake The Core Region. The other known remnants remained in the area and some attempted to make peace with humans (with varied degrees of sucess).
While many are very stupid, most are more intelligent than there east-coast cousins.