Super Mutants
Super mutants are humans mutated by FEV. They are much taller and stronger than when were humans. Their skin is mostly greenish. Their physical endurance is truly superior to any other beings in wasteland - they are completly immune to radiation and regenerate very quickly. There is however a price - most of super mutants are stupid. There are however exceptions like Marcus or Lieutenant showing what Master aimed for. All of super mutants are sterile.
History
Super Mutants are creations of Richard Grey known as Master. Master was experimenting with FEV in order to create perfect beings, well adapted to post-nuclear world. After many tries he finally succeded and created mutated humans - Super Mutants. Those creatures were supposed to repopulate wasteland and eleminate anyone opposing Master plan.
Mariposa facility where Super Mutants were being created was destroyed in 2162 by Vault Dweller. This put end to creation of new mutants.