I can only account for Marnus (which I rather like) by assuming it's a compound of
Hermanus and
Marius or
Marinus. That kind of name formation is quite usual in Afrikaans families, usually as a way of honouring two people at once.
Are you watching the Ashes? I find it rather sad but also mildly amusing that Marnus Labuschagne plays for Australia and
Dawid Malan plays for England. Right now, the South African team could use both of them ...