A Lithuanian tale, this version is taken from the 1938 anthology Wonder Tales from Baltic Wizards by Frances Jenkins Olcott.
There was once a man who had three sons, two were clever, but the youngest was simple. The Father bought each of them two horses.
One day they heard that something was eating up their barley. The first night, the Father sent the eldest son to the field to watch the barley. But he fell asleep and saw nothing.
And the next day, when he came home and his Father asked, “Now what have you seen?” he said, “Nothing.”
The second night, it was the second son who must watch, and he also saw nothing.
Now came the turn of the Simpleton. The Simpleton took a halter, went to the barley field, and sat down on a stone. There he sat till midnight. Just at midnight a White Horse came flying, and he was so white that the whole earth glowed with brightness. The Simpleton caught the White Horse.
Then the White Horse spoke:
“Set me free, and whenever you are in need I will help you. You have only to call, ‘Little White Horse!’ and at once I will be with you.”
Then the Simpleton let the White Horse run away. And when, the next morning his Father asked him, “How now, Stupid, have you caught anything?” he answered:
“Yes, I have caught a White Horse. But he begged me so hard to let him go, that I set him loose.”
One day the news came that the King would give his daughter in marriage to any lad who on horseback could leap from the castle courtyard to the third story. The Father let his two elder sons start out, Simpleton must stay at home. But he begged so hard that he might go along, that at last his Father consented.
After they started, Simpleton went off by himself, and called:
“Little White Horse!”
The Little White Horse came running. Simpleton crept into one of his ears and out at the other. And there he was, a handsome stately youth!
Then he rode to the King. Once in the courtyard, he gave the Little White Horse the spur in the flank, and sprang with him into the air, and leaped up to the third story to the King’s Daughter. The King’s Daughter gave him her ring, and he rode away home.
When he was not far from the house he left his horse. The lad now looked just as he always did.
Then he went into the kitchen and sat down by the stove where it was nice and warm, and started to examine the ring. He had wrapped up his finger on which was the ring, and now, when he unwrapped it, a bright light filled the whole kitchen. He covered the finger again, but his two brothers called out:
“Simpleton, what are you doing? You will certainly set the house on fire!”
One day after that, the King sent an invitation to a feast. The Father let his sons go, Simpleton too, and went along himself. Every one sat down to the table, except Simpleton who seated-himself by the stove.
Then the King’s Daughter passed the cups to the guests, and when she came to Simpleton, she said to him:
“Why is your finger bound up?”
She unbound his finger, and there streamed forth a bright light over all those sitting in the room. When the King’s Daughter saw that, she took Simpleton by the hand and led him to her Father, saying:
“This is my bridegroom.”
After that she led him out of the room, washed him, put on him the most beautiful clothes, led him back into the room, and made him sit down at the table beside her.
And the Simpleton and the King’s Daughter were married.