This event occurs in Chapter Four, as Ruth is first introduced to us at Hailsham when she and Kathy first begin to play together. Kathy remembers an event when she is playing in a sandbox with other girls. Ruth approaches them and tells the other girls off, then inviting Kathy to go with her to ride her imaginary horses. It is clear that Ruth has created an elaborate fantasy world with very precise methods of interacting with each of her horses, and although at first Kathy seems to get the hang of it, Ruth then turns on her angrily because of how Kathy is mistreating these animals, and tells Kathy to put the horses away.
What is key to realise is the way that this tells us a lot about the kind of character that Ruth is, and in particular the dominant personality that she is. This of course is very important when we think about how she relates with Kathy. From early on, Kathy shows herself a weak character compared to Ruth, who goes along with Ruth and is swept away by her strong character and need for control. We can therefore partly understand why Kathy never says anything about Ruth and Tommy, even when her own feelings are so much stronger for Tommy that Ruth's feelings are.