It was the mice. It was always the mice. The humans didn't know, of course. But that was part of their plan. The humans weren't supposed to know.
The medallion was part of a great experiment. But for that experiment to be a success, Jared had to lose the medallion.
After an inquiry by the MIB (Mice In Black), they decided it was time to teach Jared a lesson for interfering (albeit unwittingly) in the great experiment of Benjamin Mouse. It was Benji, as his friends lovingly called him, who after talking with poor Rob decided that turnabout was fair play. So when the jackass ran into Jared at full jackass speed, and sent the boy tumbling to the ground, it was bound to happen.
Jared forgot about the medallion in the confusion. That's when Benjamin Mouse snatched it up and disappeared into the 42nd dimension (a dimension only inhabited by the hip pan-dimensional mouse sect, of which only one human had ever discovered - a writer from England who made a pretty pound writing about his adventures with a towel and a guide book and bypasses).
Little did Jared suspect when he had entered the zoo this day that he would end up in a cage between Rob and a pig who was once an administrator, and wouldn't be leaving any time soon if the mice had anything to say about it (which they had quite a bit to say about it, as it turns out).
Benji Mouse peeked out from behind the 42nd dimension. He watched with a smug "cat-who-swollowed-the-canary" smile, as Jared scratched his arm absently as the change crept up on him. It was when he realized that he was about to change that Jared remembered the medallion, and started looking for it frantically.
"I'll show you a hairy situation, you jackass," Benji muttered, sitting back in a lounge chair that appeared beneath him and sipping on a martini. It was only fair, after all; Jared had turned into the very sort of bully that he had been tormented by for so many years. "Humans," Benji Mouse sighed. "Will they ever learn?" He watched the increasingly frantic youth as the first signs of transformation began manifesting themselves. This was going to be the best show in town, and he was going to enjoy every second of it...