Ȝ ȝ

The yogh technically did not exist in Old English, which used one sign for /g/, /G/ (a voiced velar fricative), and /j/ (which is consonantal "y" in English)--and that sign was a sign derived from the Gaelic "g" sign, which doesn't really look much like a yogh at all.

In Middle English, yogh (descended from this shape) was used to represent sounds such as /G/, /j/, /w/, and /x/ (a voiceless velar fricative, as in loch; the ach-laut). Our familiar letterform "g" was borrowed from the rest of Europe and given the French values of /g/ and /dZ/ (the consonant in "judge").

Yogh was often transliterated by {z}, just as thorn was transliterated with {y} as in "ye olde..". "McKenzie", then, is a spelling pronunciation--yogh wasn't pronounced as /z/.

Yogh is in Unicode, in the block for Latin Extended-B. Capital yogh is at U+021C, and lower case is at U+021D.

According to the dictionary, it's pronounced with a /g/ or a /x/ at the end. (Contrary to my expectations, it's the "y" that is spelled with the yogh, not the "gh".)

Source: http://www.evertype.com/standards/wynnyogh/ezhyogh.html