Hi Aldath! Thanks for reaching out! So, monsters have two opportunities to act: when a player fails or gets a mixed success during a Bout, and also during the Tempo Change and the end of each round of combat. This makes the monsters a little more dangerous and dynamic.
For small beasts, they can act instead of a monster when a player fails or gets a mixed success - for example, if someone with a bow rolls a failure on their attack and a pack of Ancorhynch are closer than the monster is, I might decide the Ancorhynch attack that character instead of the monster. I also have them act during the Tempo Change, although they typically can either move or attack, not both.