There are a total of 6 cubs you can unlock though gameplay by earning the specified type of medal in 3 of the 5 contests in the indicated location. After Obtaining at least 2 of the medals the cub will appear around the area hiding. When you earn the third medal of the same type your current cub discovers the secret cub and a short scene will play and the cub will become unlocked. This applies to ALL secret cubs except the Saber-tooth Tiger which becomes unlocked when you complete the game.
Unlock White Jaguar Cub: Earn 3 Silver Medals at La Selva.
Unlock Siberian Tiger Cub: Earn 3 Gold Medals at Suri Mountain.