He's supposedly a relative of mine who I read about in Your Darley Ancestors by Lon J Darley 1979. This website that another r/AskHistorians historian helped me locate is based on this book.
The Darley history is very interesting and can be traced back to around 1034 when Edmund Darley was born in Erle, Normandy, France. Edmund Darley assisted Duke William of Normandy (William the Conqueror) in the Battle of Hastings in 1066. One of the major factors in the battle and victory for William was the use of the Darley horses. Duke William became King William I of England and was one of England's greatest kings. Darley was granted large amounts of land and money and was knighted as Sir Edmund Darley by King William I.
I figure if he was a knight and owned land there would be some records of it. Also it would be bad ass to be related to the guy that William the Conqueror gave props to in helping him conquer England.
The author also claims that his son was a knight Sir John Darley Born ca 1066 Erle, Normandy. Also I know that the Darley Arabian is a super important horse breed This source claims that 95% of thoughorbred race horses are related to one Darley Arabian horse. So maybe the history of the breed would be connected to this origination story?
Edit: Just to clarify the title on this is wrong the horse breed at the time was the Darley, it became the Darley Arabian after it was bred with a horse Thomas Darley bought in Aleppo in like 1700.
There's a few issues with this story that lead me to believe it is likely a fabrication, or at least a great exaggeration, as family genealogies are prone to.
First, I can find no reference to an Edmund Darley in primary or secondary sources outside of the Darley family history. Nor can I find an Erle in Normandy.
Second, at this point in European history surnames really didn't exist. What passed for surnames were mostly geographical or occupational names - for example, William d'Hauteville means William of (the village of) Hauteville, or Thomas the Butcher, etc. Or they were nicknames that references some character trait. Robert Guiscard means literally Robert the Wiseass. Were your ancestor a nobleman from Erle, one would expect him to be called Edmund D'Erle, not Edmund Darley. Darley is a very stereotypically Anglo-Saxon sounding place name, so I would expect the family originates from there, not Normandy.
Third, horses were exceedingly expensive animals; just one could equal the yearly profit of a small village. Any man who owned multiple horses - enough that he could loan them to a duke in appreciable numbers - would be a major landholder, a viscount at the least. If so, there would be no need to knight him, because he would, presumably, already be one. Knighthood was not yet as rarefied as it would become in later centuries; if you could afford horse and armor, you were entitled to be a knight.
Fourth, I'm not aware of any evidence that discreet breeds of horses existed at this point. Medieval horses were generally bred to a type, rather than a breed - a palfrey was a light riding horse, a rouncey an all-rounder, a courser a strong, swift war horse, a destrier a heavy tournament horse, etc. Even this types are not attested to for the 11th century; I suspect horses at this time were less specifically bred. I've certainly seen no reference to the Darley in the 11th century.