Will Danoff, one of the U.S. mutual fund industry's best stock pickers over the past 25 years, is tapping the brakes on funding so-called unicorn companies, saying the prospects for fast growing private businesses before they go public may be fading.

Unicorns — start-ups valued at $1 billion or more — are making front-page news around the globe, and that makes Danoff nervous. He runs the $111 billion Contrafund for Boston-based Fidelity Investments and his pre-IPO investments can lend an imprimatur of legitimacy, given his track record of picking blue-chip winners.