After you beat the game, you unlocked Zongazonga's Castle. Once you go in there, Talk to the the staff and pay 3000 G.

Unlockable	How to unlock
Z-Elasmo	Beat Zongazonga Again.
Z-Ptera	Beat Zongazonga Again.
Z-Rex	Beat Zongazonga Again.
Z-Tricera	Beat Zongazonga Again.
Zongazonga	Beat Zongazonga Again.