Tony assumedly killed the Chimera a long time ago, but now everyone is upset that it is back.
After The Recently Escaped arrived, there are multiple hedge gates inside of the zoo that anyone could use to enter / exit the Hedge, which is also where the Chimera resides.
Before it was interrupted, the Freehold agreed to lead a joint Autumn Summer investigation into the Chimera.
The issue of the Chimera was put on the backburner, and with the uncertainty of the current Summer monarch, Michaelwishes to defer action until such time as it is necessary to preserve life.
Michael broaches the issue of the Chimera again in the wake of all the other drama happening in the Freehold. He says that any of the older changelings such as Tony, Lyre, Tommy, or Jim would know about the first time the Chimera was around, but that it was before his time as King of Autumn.
Tony has also said that Sam would have known something about the Chimera but, well... you know...