What's the significance of the title of "The Rocking-Horse Winner"?
The title is ironic. Paul can determine which horse will win an upcoming race by riding his rocking horse, so in that sense he is a winner. He wins an enormous amount of money for his mother by furiously riding the wooden horse until it becomes clear to him which real horse to lay a bet on.
However, he is not really a winner at all, because the obsessive riding kills him. Further, although he is too young to understand this, no amount of money could ever win him what he really desires, which is his mother's love. She is empty inside, incapable of loving. He believes, in his young way, that enough money will fill her up, but, in fact, the more she gets, the more she wants. So while Paul wins bets, in the end, nobody in this story really is a winner.
Without the rocking horse, young Paul would not be blessed with the good luck that, according to his mother, has always avoided his father. Although Paul has outgrown his childhood toy, he somehow determines that riding the horse will provide him with the winners of the local horse races. He does not predict the winners randomly or by himself; they come only while he secretly simulates the actual races on the rocking horse, and he apparently always picks (or the wooden horse whispers to him) the winner of the race. Critics claim that the horse also serves as a symbolic sexual device, as Paul works himself up to a furvor while wildly riding the wooden horse.