Dian is a variant of Diane — from the Roman goddess Diana, Latin divus (divine). Her direct-action conservation against poachers — she called it "active conservation" — earned her enemies; she was murdered in her cabin on December 26, 1985, the case officially unsolved. The Dian Fossey Gorilla Fund continues her work.
Dian does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Dian reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.