The investors on the BBC programme Dragon's Den are called dragons.
The name comes directly from the programme title: each entrepreneur enters the den to face a panel of wealthy business investors, who are collectively known as the dragons. They listen to pitches, question the founders and may offer money in return for an equity stake.
The term is part of the British version’s identity and differs from the animal imagery used by related international adaptations. In the United States, the comparable investors are called sharks on Shark Tank. The Japanese source format Money no Tora uses the image of tigers in its translated title.
The plural answer is “dragons,” not the name of a particular investor. The programme has had changing panels over the years, but the collective label remains stable. This terminology is why the British title refers to a den rather than a tank.