The S&P 500 index is a collection of 500 large-cap companies that represent "leading companies in leading industries." The index oftentimes is a benchmark of the overall U.S. stock market and is also part of many investors' portfolios through index funds and SPDRs. While the S&P 500 index lost 16.8% in the month of October, its bottom 10 companies have seen their stock prices fall even further.

Of course, a stock's price can drop for reasons both significant (e.g., the emergence of a powerful competitor) and insignificant (e.g., tax selling). Hence a large drop in stock price could offer a unique buying opportunity, but it could also present a value trap.

That's why we've paired the 10 biggest S&P 500 losers for the month of October with the intelligence of our 120,000-member-strong Motley Fool CAPS community. Each company's CAPS rating should offer some insight into how our community views the company. As always, though, you should conduct your own fundamental research.

For the month of October, here are the S&P 500's 10 biggest market losers.

Company

Return in October

Year-to-Date Return

CAPS Rating
(5 max)

Additional CAPS Research

1. Hartford Financial Services Group (NYSE:HIG)

(74.8%)

(88.2%)

**

HIG

2. General Growth Properties (NYSE:GGP)

(72.6%)

(89.9%)

*

GGP

3. ProLogis (NYSE:PLD)

(66.1%)

(77.9%)

***

PLD

4. Lincoln National Corp.

(59.7%)

(70.4%)

***

LNC

5. Coventry Health Care (NYSE:CVH)

(59.5%)

(77.7%)

*****

CVH

6. Developers Diversified Realty (NYSE:DDR)

(58.4%)

(65.6%)

*

DDR

7. Prudential Financial (NYSE:PRU)

(58.3%)

(67.8%)

**

PRU

8. Apartment Investment & Management

(58.2%)

(57.9%)

*

AIV

9. Ford Motor Co. (NYSE:F)

(57.9%)

(67.5%)

*

F

10. Principal Financial Group

(56.3%)

(72.4%)

***

PFG

Source: Capital IQ, a division of Standard & Poor's, and Motley Fool CAPS.

Join us on CAPS to further research these companies and share what you know with the community.

The Fool has a disclosure policy and it's not a loser at all. Coventry Health Care is a Motley Fool Stock Advisor pick.