John m. Latin [I/J]o(h)annes, from Greek Ἰωάννης, from Hebrew יוֹחָנָן 'graced by God'.
The name of the Baptist and one of the Evangelists, twenty popes and two anti-popes, a 4th C martyr, three 4th C saints, a 5th C Greek saint, a 5th C Armenian saint, three 7th C saints, an 8th C saint, a 9th C saint, a 10th C Byzantine emperor, a 10th C Bulgarian saint, an 11th C Italian saint, a 12th C Byzantine emperor, four 12th C Italian saints, a 12th C French saint, two 13th C kings of Jerusalem, a 13th C king of England, a 13th C king of Sweden, a 13th C king of Scotland, two 13th C Byzantine emperors, a 14th C king of Bohemia, a 14th C king of Portugal, two 14th C kings of France, a 14th C king of Castile, three 14th C Byzantine emperors, a 14th C Greek saint, a 14th C Czech saint, a 15th C king of Portugal, a 15th C king of Sweden, a 15th C king of Castile, a 15th C Byzantine emperor, a 15th C Italian saint, a 15th C Polish saint, a 15th C Spanish saint, a 15th/16th C king of Denmark, Norway, and Sweden, a 16th C king of Portugal, a 16th C king of Poland, a 16th C king of Sweden, a 16th C English saint, a 16th C Spanish saint. Wycliffite Bible (1395): Joon.
From about the 12th century, John in all its variants and diminutive forms became one of the most popular masculine names in Europe, in many times and places eclipsing the next most popular man's name by a factor of two.
Evan is a Welsh form, popular in England in counties that bordered Wales, such as Shropshire. The standard Gaelic spelling is Eoin.
Diminutives in -co and -ko are indicative of Slavic influence.