(Adds details on other funds' returns, quote)
By Svea Herbst-Bayliss
BOSTON, Jan 5 (Reuters) - Billionaire investor William Ackman's Pershing Square Holdings hedge fund ended 2015 with a 20.5 percent loss after Valeant Pharmaceuticals, a top holding, was battered in the second half of the year.
That put the closely watched activist investor into the same league as rival billionaire hedge fund manager David Einhorn, whose Greenlight Capital finished also ended the year with a 20 percent drop, according to investor updates seen by Reuters.
For Ackman, the drop marks the worst performance in his firm's 11-year history. For Einhorn, it was the first down year since his fund lost 23 percent in 2008, when many hedge funds were in the red during the financial crisis.
Pershing Square Holdings, one of several portfolios run by the Ackman's $14.9 billion hedge fund, ended December nearly flat with a 0.3 percent gain for the month, the update showed.
Einhorn's roughly $11 billion fund posted a similar return, inching up 0.4 percent in December, the update showed.
Pershing's loss had been expected since Ackman had been hurt by accelerating falls in big name stocks since the summer months. But it also illustrates just how volatile the activist investor's returns can be after he scored a 40 percent gain in 2014 when he ranked among the industry's best performers.
Valeant hurt Pershing Square while Greenlight was hit by bets on Consol Energy and Micron Technology.
Barry Rosenstein's Jana Partners fund lost 5.4 percent last year, falling on bets on semiconductor company Qualcomm and car rental group Hertz Global.
Even though 2015 is expected to be remembered as another disappointing year, some funds sidestepped disasters and finished the year with sizable gains.
Computer model-driven firm Renaissance Technologies' $5.5 billion Institutional Diversified Alpha fund gained 16.2 percent, while the firm's Renaissance Institutional Equity Fund finished the year with a 17.3 percent gain.
Jacob Gottlieb's Visium Asset Management's $2.5 billion Visium Global fund ended up 10.3 percent.
Hedge funds have come under "scrutiny from investors, as returns from the industry as a whole have simply not been on par with expectations," wrote Stanley Altshuller, chief research officer at investment services firm Novus Partners. "But the averages hide many positive outlying managers who were able to identify massive opportunities and deliver solid returns."
Reporting by Svea Herbst-Bayliss; Editing by Alan Crosby