5,000,000,000 Yen
Whilst in the stables, press B to enter the main menu, then enter the last option (save). Press Select to enter the "TURBO FILE" menu, then press Right, Up, Left, Left, Down, Up, Down, A. After the message about loading, another message will appear, indicating you have now five billion yen.
Revive Breeding Mare
When a breeding Mare dies, you get a message in Japanese. While this message is being displayed, press Reset and select Continue. If done right, the Mare will be alive and the game will start on May of that fiscal year.
See The Ending
At the title screen, on controller 2, hold Up & B and wait until the song ends. When it does, the ending will start.
View Horse Stats
Whilst in the stables, select any horse from the left part of the field. Then, at the info screen, press Select, Left, Up, Left, Up, Right, Up, Start to see all numerical parameters of the selected horse.
5,000,000,000 Yen...