Yogh was an insular form of the letter "g" commonly found in Old English manuscripts. In appearance yogh was similar to the number "3". The name "yogh" is thought to derive from Old English words for "yew" or "yaw". When the French introduced the letter g after the Norman Conquest yogh shifted meaning and was used during the Middle English period to represent the sounds of the modern letters "gh" (as in "li3t"), "y" (as in "New 3ear"), and "z" (as in the name "McKen3ie"). Yogh gradually faded out of use in the late Middle English period because the English language already was using the letters "y" and "z" and "gh" was becoming more common, making yogh no longer necessary.

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