Beasts and monsters can still evolve even without the help of a tamer. However, this process usually takes an extremely long time. They need to store enough energy for evolution one drop at a time, which takes years, if not decades, to reach the next stage.
However, there's a boundary that normal beasts and monsters can't overcome. The buffer zone; the 8th stage of evolution.
Only the chosen ones, or those blessed with special constitutions and talents, could surpass the buffer and complete their evolution to Duke or Duchess level. However, this undertaking usually comes with a heavy price.
"Duke of Unsatiable Hunger..."
I muttered the name of this famous beast, responsible for the fall of several frontline bases around the territory. I have heard of him through a lot of hearsay and even saw his image in a newspaper somewhere before my past memory returned. It was the harbinger of death for most tamers on the frontlines.