A Problem Tree is a causal map disciplined into a near-hierarchy. Small, concrete issues are linked upward through cause-and-effect chains into fewer, more abstract, top-level problems - wikis.ec.europa.eu 
The “dragons” are those top-level peak problems that: 1. Summarise many underlying causes. 1. Are broadly recognised by different stakeholders, even if they disagree about details. 1. Are distinct enough to name clearly without overlapping too much. 1. Feel large and serious, but still connected to causes that someone in the coalition can influence.
In workshop reality, you often end up with around four or five dragons because attention is limited and coalitions need a small shared story to act together. Of course it may just be that dragons like busses come alone or in small groups.
# See