Sometime in the course of human evolution a side-branch emerged who called themselves the Neo. They were happy with their solitary life in their community which initially prospered, until tragedy struck. Due to the actions of the High Evolutionary (in Uncanny X-Men #379-380) temporarily "turning off" the mutant gene to transform all mutants into "baseline" humans, many of the Neo perished. Among the deceased was the daughter of their leader Domina, who could breathe underwater and drowned when her powers were removed. Domina and other Neo who survived swore vengeance (misguidedly) on the X-Men and Mister Sinister (who Domina has "killed" on several occasions), with those leaving their home to exact their revenge now calling themselves the Warclan.
See Neo for more.