I always imagined that they were just generally rounded up by the Guardian masters. After all, Garden does seem to control the Fire Cavern so it didn't seem implausible to me that they might've rounded up some monsters to do their dealings for them. As for the Grendel, I just assumed it might've been kept somewhere in Garden for specialist training or something. After all, the Training Centre would be pretty rubbish if it only contained Grats and T-Rexaurs.

Or y'know a wizard did it.