Solution:

The solution for the puzzle is that of the two horses drawn the head and back of each is not the one linked with lines. The head for the first horse is the back of the horse on the other side of the card and vice versa. To solve the puzzle the two horse pieces are placed in a way that the back of the horse on the first piece is facing the back of the horse on the second piece. In the gap between should be slid the jockey's piece of paper, thus forming an image on which a horse is running to the left and the other to the right, one upside up, and the other upside down.

Do you know?

Famous Trick Donkeys is a puzzle invented by Sam Loyd in 1858, first printed on a card supposed to promote P.T Barnum's circus. P. T. Barnum saw the opportunity to promote his show on this puzzle card and offered ten thousand dollars to Sam Loyd to change the name of the puzzle to "P.T. Barman's Trick Mules".

Later on, and after Loyd had offered the puzzle to other firms, it was renamed again to "Famous Trick Donkeys", which sold more than 100,000,000 copies. Today The Sam Loyd Company owns the rights to the puzzle and released a commemorative puzzle card in 2016, 145 years after it was first published.