Former Speaker Joe T. San Agustin
The last name San Agustin does not appear in any Spanish records in the Marianas till the 1800s.
By that time, contact with Mexico was gone, since the war for Mexican independence began in 1815. That was the year the last Acapulco galleon sailed for Manila.
After 1815, the bulk of all outside contact with the Marianas, save for the whalers, was through Manila, and more and more Filipinos settled on Guam.
The first San Agustin to move to Guam could have come from many places, but the Philippines would be a good guess.
It seems that he had five children. We can account for two sons, Vicente and Mariano, and they both had the middle name Tainatongo, believed to be an indigenous, Chamorro name. Today, one thinks of Malesso' when they hear Tainatongo, but the family actually was from Hagåtña and some moved down to Malesso' years later.
Vicente Tainatongo San Agustin married Juana Crisostomo, the daughter of Maria Crisostomo. Her father seems to be unknown. They had a good number of children.
Mariano Tainatongo San Agustin married Maria del Espiritu Santo. I don't know Maria's parents. They, too, had a good number of children.
All the other San Agustins in the 1897 Census are women (three) so it seems that these two males, Vicente and Mariano, are the male ancestors of the San Agustins of Guam.
Though a small clan and of recent foundation, the San Agustins have made their impact on the island, producing civil servants, government heads, a laicized priest* and a teaching sister/principal.
* Laicized means a priest who no longer functions as a priest and returns to the lay state.