Just who was St. John Lateran?

Today's feast raises the annual question, "Just who was St. John Lateran anyway?" Nobody seems to know this obscure saint. We know of John the Baptist and John the Evangelist, and some may recognize the names of John Chrysostom, John Vianney and even John Bosco. But this Lateran guy?
You are forgiven.
Today's feast is not a saint's day at all. It is one of those celebrations that doesn't fit easily into one's devotional life: the feast of the dedication of a Church. Sort of like a Catholic Hanukkah, except that this feast commemorates the original dedication, not the rededication of a place of worship. There are two such feasts on the universal Catholic calendar. (Ten points if you can name the other one! And twenty bonus points if you know which other feast on the universal calendar developed out of the dedication of a third Church--add ten points if you know which church.)
Today's observance is the dedication of the Lateran Basilica, which is the actual Cathedral of the Bishop of Rome (you know, the Pope). The Lateran Basilica is about two miles, maybe less, from the Vatican Basilica, better known as St. Peter's, and just down the road from the Liberian Basilica, better known as St. Mary Major. The name "Lateran" comes from the name of the Roman family who originally lived on that bit of property. (Constantine took it and eventually gave it to the Bishop of Rome.) The real name of this cathedral is the Archbasilica of the Most Holy Savior and of St. John. And if you ask (again!) which St. John, it turns out to be two of them: the Baptist and the Evangelist. But neither of them, it turns out, was a Lateran.