Chrono Trigger isn’t turning into Chronomon. You aren’t going to wander through time trapping monsters for the monster arena feature introduced in the DS release. The four Nu-like creatures pictured on the top left are smidges, baby monsters assigned to one of the four elements in the world of Chrono Trigger. The stable master in the Arena of Ages gives you a smidge of your choice for free and the infantile creature can be trained by sending it to different time periods. The Middle Ages era boosts the strength attribute.
Once you feel ready your monster can be sent into the arena to fight computer controlled monsters or a friend through DS wireless play. Monsters automatically attack when their time gauge fills, but you can alter the attack they use. Items like the Hawk Talon, Seafang, and Duskeye which activates shadow-based attacks give players limited control of their monsters. It sounds like the monster arena opens early since Square Enix mentions you can “unlock eras” to further train your monster. One thing is clear, you will be able to enter the Arena of Ages before you reach the Kingdom of Zeal.
Images courtesy of Square Enix.