Thousand-league colt

[NewHope]

Thousand-league colt = ?

His father loved his son dearly and had high hopes on him.
"This," he would say, "is the thousand-league colt of our family."

[AintFoolin]

i've never heard the phrase and neither has google (only link was to a japanese page strangely enough)

but the meaning is clear enough, as the sentence said, he was the one they had high hopes for

they are make some sort of comparison to a prize horse

[Reemy]

There is an old use of the word "league" means a unit of measuring distance = 3 miles or 4000 m.
Thus , the son is good enough to be compared to a high-quality horse that can win a 3000 mile race.