The wealthy investors on the British television show Dragon's Den are usually called Dragons.
The title supplies the programme’s central metaphor: entrepreneurs enter the den and present their businesses to powerful investors. A Dragon may ask questions, challenge the valuation, make an offer or declare that they are out of the deal.
The label is specific to the British branding and to related versions that retain the Dragon's Den name. The American adaptation uses “sharks” instead, which is why viewers sometimes give the wrong answer when comparing the two programmes. The Japanese source format used another name again.
A Dragon is not simply a presenter or judge. The people in these seats are investors who can potentially provide money, advice and business connections in exchange for equity. The dramatic format turns that commercial negotiation into a television episode.