'Yogh', pronounced "yog", is a letter in old English and Scots which has no exact equivalent today. It used to be written a bit like the old copperplate-style "z" (it looked like a 3), helps explain the discrepancy between the spelling of Menzies and the pronunciation "Mingis".
The rise of printing in the 16th Century coincided with the decline of the yogh, and so it tended to be rendered in print as a "z", and pronounced as such.
(This means that newsagent John Menzies is more properly pronounced John Ming-iss.)
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