Ardebaran:

To get the ultimate moster you must be a master breeder and have commitee permission. When you are about to breed a new monster, insert the PSX game

Monster Rancher Megamix :
If you stick your Monster Rancher game into a regular CD player you can listen to some pretty good techno music hidden on the disc.

Loyalty trick:
Keep bringing your monster from the ranch to the town. Each time you return to the ranch your loyalty goes up 2 or 3 percent, but your breeding style gets a bit stricter.

Tecmo's Deception to create a secret monster called Ardebaren.