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.
tipster in British English
(ˈtɪpstə)
noun
a person who sells tips on horse racing, the stock market, etc
tipster in American English
(ˈtɪpstər)
noun
Informal
a person who sells tips, as on horse races or for stock speculation
Examples of 'tipster' in a sentence
tipster
His official profession was that of racing tipster, an occupation which permitted him to mix with the low life of the city unnoticed.
Forbes, Colin COVER STORY (2001)
She wondered if there was a way to tell her anonymous tipster to direct all future correspondence to her office.