Someone who makes their living by studying the form of racehorses and selling his advice on wagers that he considers prudent to other punters
One who makes a practice of giving or selling tips, or private hints or information, esp
A tipster is someone who tells you, usually in exchange for money, which horses they think will win particular races, so that you can bet money on the horses. someone who gives information about which horse is likely to win a race
one who sells advice about gambling or speculation (especially at the racetrack)