Sir Thomas appears to have been a cadet of the House of Errol, as his descendants are mentioned in the table of succession to the Errol title.
The Hays of Errol trace their ancestry back to William Hay who, ploughing near the battlefield of Luncarty in 980, saw the King of Scots,
Kenneth III and his earls fleeing after an unsuccessful brush with Danish invaders.
Taking the yokes from the necks of their oxen Hay and his two sons stood in the way and halted the rout.
Once brought to a standstill, King Kenneth considered the situation, and reinforced by the three Hays, returned to the charge.
The Danes, who by this time were enjoying the plunder, were taken by surprise and utterly defeated.

The King rewarded Hay with a knighthood, and granted him as much land as his falcon would fly over without alighting.
The arms of Errol with the three shields, the falcon crest, and the supporters bearing ox yokes, commemorate the occasion.
At Yester a massive silver candelabra showing William Hay grasping the reins of King Kenneth's horse was made in India and
presented to the 8th Marquess by the people of Madras.

A charter of William the Lion granting the lands of Locherworth to Sir Thomas Hay is still at Yester. Part of the Barony of Locherworth went to
Lord Borthwick on his marriage to a daughter of the house in 1438 and the estate is now called Borthwick although the Barony
remained in the hands of the Hays until 1580.

The title ' Tweeddale' derived from the lands in Peeblesshire brought to the Hays by marriage to the heiress of Simon Eraser of Oliver Castle,
known as 'The Patriot,' who was executed by Edward I in 1305. These lands included Oliver Castle and Neidpath.
As all Simon's sons were killed in battle the Hays quarter the arms of Eraser and the head of the family is chief of Clan Eraser.
At the same time the Hays became Hereditary Sheriffs of Peebles.

Hay of Yester was created a peer in 1488. From then until the 17th century the Hays were a normally turbulent Border family; loyal to the King,
but not above a bit of local violence. One of them was killed at Flodden, while another endowed St. Cuthbert's at Bathans as a Collegiate Church.
The Master of Yester, heir to the title, was ' Put to the Horn,' i.e. declared an outlaw, for murdering a neighbour, and yet another lost
his office for allowing a condemned prisoner to escape.

In 1540, during the Rough Wooing, when Lord Somerset brought an English army to try to persuade Mary de Guise to betroth the infant
Mary Queen of Scots to Edward VI the Castle of Yester was besieged and eventually surrendered to the English.
A letter from the French ambassador to the Pope complains that while the Scottish garrison were allowed to march out with the honours of war
their French allies were stripped and beaten, and some Spaniards among them, murdered.
It was during this campaign that the abbeys of Haddington, Melrose, Dryburgh and Kelso were destroyed.

As this Lord Yester had no son he left his estates to his brother James, but he died suddenly before the document of entail could be sealed.

James had to ask Lord Chancellor Maitland to affix the seal in arrears or lose his estates.
Maitland agreed to do so but demanded the lordship of the estate of Lethington (Lennoxlove) which he had held in feu from Yester in return for the favour!

In 1594 Broun of Froisthill complained that Lord Yester had 'caryit him to his place of Neidpath and put him in the pit thereof.' Hay was ordered
to release his prisoner within six hours on pain of being put to the horn. The next year Lord Yester's page, one Hepburn,
accused his Master of the Horse, Brown, of offering him a reward to poison their master.
The business was brought before the Council, and as Brown denied all knowledge of the plot, the matter was decided by a trial of combat.
They fought, first on horseback and then on foot, in front of an impressive panel of judges, and *as Brown was left for dead on the field,
he was judged guilty. This was the last Judicial Combat to be held in Scotland.

In 1584 the house of Bathans was built on the present site; the date, with the initials of William Hay and his wife, Margaret Kerr,
is carved on the mantelpiece of the Laigh Hall of Yester, which must have been the Great Hall of Bathans.
The 8th Lord Yester was a Privy Councillor to Charles I and in 1646 was created an Earl by him as Earl of Tweeddale.
His mother endowed Lady Yester's Church in the Canongate, Edinburgh, and a parchment exists which gives the family a perpetual right to a pew there.
Lady Yester was the daughter of the Earl of Dunfermline. Charles I was born at Dunfermline and he was a sickly child who suffered from rickets.
His mother, Anne of Denmark, left him to be brought up by Lady Yester's stepmother, the Countess of Dunfermline.
A portrait of him as a child in a sitting-room at Yester shows him walking with a stick; it was probably a gift from him to his foster sister.

The 2nd Earl was the first Hay of Yester to play a leading part in Scottish affairs. During his father's lifetime he held Neidpath against Cromwell
and it was the last place south of the Forth to surrender to the English.
Cromwell is also supposed to have lost some of his guns crossing Daneskinbog, to the east of Yester.
A local legend also credits this bog with the defeat of the Roman ' Lost Legion' — the 9th Hispania — which disappeared without trace after
marching out of Trimontium (near Melrose) in the 2nd century A.D.
In spite of his resistance to Cromwell Lord Tweeddale sat as an M.P. for East Lothian in the Lord Protector's Parliament from 1656 to 1660.
At the restoration he hurried to greet Charles II and was appointed by him to the Privy Council. He was more or less at the head of the Scottish Administration
from 1667 until 1675, but lost his office in that year owing to the enmity of the Duke of Lauderdale with whom he had been until then on the best of terms.