Mistings are Allomancers who can only gain power from one of the Allomantic metals. All Mistings in the Final Empire are the decedents of the people who ingested Larasium around 1000 years ago. Larsium granted powers far more potent and varied to those individuals, but time diluted the Allomantic potential in individuals.
Alendi is the earliest Misting (a Seeker) known to us, although Sazed states other Mistings existed at that time. These Allomancers existed before Larasium, and their Allomancy seemed to follow different rules. For example, there is no record of Alendi needing to burn bronze in order to sense the Well of Ascension.