Margaret Stewart, Countess of Angus
Margaret Stewart, Countess of Angus and Mar( died 1417) was Countess of Angus and Lady of Abernethy in her own right.
Her father was Thomas Stewart, 2nd Earl of Angus.
She was the widow of Thomas, Earl of Mar.
By an illicit affair with William Douglas, 1st Earl of Douglas, husband of the Earl of Mar's sister, she became the mother of George Douglas, 1st Earl of Angus( c. 1380- 1403), and secured a charter of her estates for her son, to whom in 1389 the title was granted by King Robert II.
Thomas Percy, 1st Earl of Worcester
Thomas Percy, 1st Earl of Worcester, KG (134323 July 1403) was an English medieval nobleman and naval commander best known for leading the rebellion with his nephew Henry Percy, known as 'Harry Hotspur', and his elder brother, Henry Percy, 1st Earl of Northumberland.
Patrick Ruthven, 1st Earl of Forth
Patrick Ruthven, 1st Earl of Forth and 1st Earl of Brentford( c. 1573 – 2 February 1651) was a Scottish nobleman, general, and diplomat.
Minamoto no Chikako
She was the mother of Prince Morinaga.
Waleran de Beaumont, 1st Earl of Worcester
Waleran de Beaumont, Count of Meulan, 1st Earl of Worcester( 1104 – 9 April 1166, Preaux), was the son of Robert de Beaumont, 1st Earl of Leicester and Elizabeth de Vermandois, and the twin brother of Robert de Beaumont, 2nd Earl of Leicester.
He is not referred to by any surname in a contemporary document other than' Waleran son of Count Robert'.
Humphrey de Bohun, 2nd Earl of Hereford
Humphrey( IV) de Bohun( 1204 – 24 September 1275) was 2nd Earl of Hereford and 1st Earl of Essex, as well as Constable of England.
He was the son of Henry de Bohun, 1st Earl of Hereford, and Maud FitzGeoffrey.
Henry Somerset, 2nd Earl of Worcester
Henry Somerset, 2nd Earl of Worcester( 26 November 1549) was an English nobleman.
He was the son of Charles Somerset, 1st Earl of Worcester and Elizabeth Herbert, 3rd Baroness Herbert.
On his father's death on 15 April 1526, he succeeded as the second Earl of Worcester.
From his mother, he inherited the title of Baron Herbert.
Somerset obtained Tintern Abbey after the Dissolution of the Monasteries.
He married twice:
Henry Percy, 1st Earl of Northumberland
Henry Percy, 1st Earl of Northumberland, 4th Baron Percy, titular King of Mann, KG, Lord Marshal (10 November 134120 February 1408) was the son of Henry de Percy, 3rd Baron Percy, and a descendant of Henry III of England.
His mother was Mary of Lancaster, daughter of Henry, 3rd Earl of Lancaster, son of Edmund, Earl of Leicester and Lancaster, who was the son of Henry III.
Robert Stewart, 1st Earl of March
Robert Stewart, 1st Earl of Lennox then 1st Earl of March( c. 152229 March 1586) was a Scottish nobleman of the family of Stewart of Darnley.
Simon of Worcester
Simon was a medieval Bishop of Worcester.

Question: Who is Thomas Percy, 1St Earl Of Worcester's mother?
Answer: Mary of Lancaster