Mastodons diverged from the order Proboscidea before the radiation of Elephantidae and so are, at best, distant cousins to the group.

Proboscidea's first "classified" representative was a small island mammal about the size of a county fair hog with a prehensile nose. After migrating to the mainlands the order grew large to be big and fat and worthy of their expansive new accommodations.

The mastodons were an early outgroup of the Proboscidea line which led to the latter day mammoths and elephants.
The only fossil record known of mastodons is of the American mastodon (mammut americanum)... I think.

One of the last mammoth species to exist on Earth were the pigmy elephants who lived on an island about seven miles off the coast of California. These Island elephants grew down to be about the size of a fair-sized county hog and then they went extinct.

Shame, they would have probably made great yard pets.