There are many monsters creeping in on us. Anxiety, anger, fear - you name it. Sometimes it's easier to separate that negative aspect, in order to process actions and consequences. This is specially useful with children, because we don't want to shame them when they throw a tantrum. Regulating emotions is actually really hard, they haven't learned coping skills or appropriate ways to manage their stress and frustrations.
Think about it, did anyone taught you how to regulate emotions as a child? Unless you were in therapy, is highly unlikely. Many adults still haven't learned healthy coping skills. Which is why many end up abusing substances or engaging in unhealthy behaviors, such as smoking or over-eating.
What to do?
Ask the child to draw their inner monster. You can adapt this to identify the specific emotion they are struggling with. Like draw your anxiety monster or your angry monster.