Mormaer Beth is a name of a Mormaer

Mormaer

The title of Mormaer designates a regional or provincial ruler in the medieval Kingdom of the Scots. In theory, although not always in practice, a Mormaer was second only to the King of Scots, and the senior of a toisech.-Origin:...

 mentioned in an unreliable charter granted to Scone Priory, later Scone Abbey

Scone Abbey

Scone Abbey was a house of Augustinian canons based at Scone, Perthshire , Scotland. Varying dates for the foundation have been given, but it was certainly founded between 1114 and 1122....

, by king Alexander I of Scotland

Alexander I of Scotland

Alexander I , also called Alaxandair mac Maíl Coluim and nicknamed "The Fierce", was King of the Scots from 1107 to his death.-Life:...

.

The charter (Lawrie XXXVI) says, merely, Beth comes (i.e. Mormaer Beth). This could be a mistake for Heth, a form often used for the Gaelic name Áed, or perhaps MacBethad, or even a real name as Beth, meaning life. Alternatively, the name may have been made up by either the scribe or his overseer. The only reason for associating Beth with Fife

Fife

Fife is a council area and former county of Scotland. It is situated between the Firth of Tay and the Firth of Forth, with inland boundaries to Perth and Kinross and Clackmannanshire...

 is that he appears first in the witness list, an honour often but certainly not always given to the Mormaers of Fife amongst the other Scottish Mormaers.

It is more probable that Beth this is the same person as Áed, either Mormaer of Moray

Mormaer of Moray

The Mormaerdom or Kingdom of Moray was a lordship in High Medieval Scotland that was destroyed by King David I of Scotland in 1130. It did not have the same territory as the modern local government council area of Moray, which is a much smaller area, around Elgin...

 or Mormaer of Ross, attested in two early charters of David I

David I of Scotland

David I or Dabíd mac Maíl Choluim was a 12th-century ruler who was Prince of the Cumbrians and later King of the Scots...

. Áed disappears from the record ca. 1130. His identification as the ancestor of the MacHeths

MacHeths

The MacHeths were a Gaelic kindred who raised several rebellions against the Scotto-Norman kings of Scotland in the 12th and 13th centuries. Their origins have long been debated.-Origins:...

 is uncertain.

Conflation with Ethelred of Scotland

Ethelred of Scotland

Ethelred was the son of King Máel Coluim III and his wife Margaret, the third oldest of the latter and the probable sixth oldest of the former. He took his name, almost certainly, from Margaret's great-grandfather King Æþelræd Unræd, or Ethelred the Unready...

is spurious, and based on the unlikely idea that Ethelred ever was Mormaer of Fife.