I'd assume that his surname would be given as "son of Mogh" (which in Klingon would be Mogh puqloD
, as far as I can tell). After all, there are plenty of similar "son of" surnames in human languages, like Jackson, McAndrew, O'Brien, ap Hugh, etc.
And Spock does have a family name, according to "This Side of Paradise" and "Journey to Babel," but it's very difficult for humans to pronounce. The novel Ishmael
claimed it was S'chn T'gai, while the fan-published U.S.S. Enterprise Officer's Manual
claimed it was "Xtmprszntwlfd."