The investors on the BBC programme Dragon's Den are called dragons.
The title is built into the programme’s name and is the British format’s distinctive term for the businesspeople who hear pitches. Entrepreneurs enter the Den, present their proposals and ask the dragons for investment in exchange for equity.
The label reflects the intimidating image of wealthy, powerful investors guarding their money. It is not a professional qualification and does not describe a legal category of investor. Each dragon is an individual entrepreneur or businessperson with a separate career, investment style and commercial speciality.
The main international comparison is Shark Tank, where the investors are called sharks. That difference is why “sharks” is a plausible but incorrect answer for the British programme. The Japanese source format, Money Tigers, uses another animal metaphor, but British viewers refer to the panel as dragons.