"Bets on the horses" means gambles on horse races. Are you familiar with horse races?
What does it mean "bets in the horses"?
I've got a friend who constantly speculates on the stock market and bets on the horses.
Yes, 'bets on the horses".
To tell you the truth, no.
I think one does not have to be familiar with horse races to understand the meaning of 'bets on the horses'.
I am not a teacher.
A related term you may hear in the US is 'play the ponies', meaning to wager on the results of horse races, especially somebody who does so on a regular basis.
Wear short sleeves! Support your right to bare arms!
Just for fun, I am posting a video of Secretariat's Belmont win. It was one of the most impressive horse races of all time and won by possibly the greatest thoroughbred horse of all time. He won by 31 lengths (248 feet, 76 meters).