* Wall St set for first monthly decline since August
* Mattel shares plunge after fourth-quarter results
* Wal-Mart cuts outlook; shares off in premarket trade
* Futures down: S&P 17 pts; Dow 158 pts; Nasdaq 16.75 pts
NEW YORK, Jan 31 (Reuters) - Wall Street was poised to tumble at the open on Friday and was set for its first monthly decline since August, hurt by weaker-than-expected inflation data in the euro zone and ongoing concerns about turbulence in emerging markets.
* Stock index futures declined, and the Dow Jones industrial average and the S&P 500 were set to open about 1 percent lower.
* Inflation in the euro zone slowed this month to 0.7 percent from 0.8 percent in December, confounding expectations for a rise to 0.9 percent and matching a low hit last October. The European Central Bank responded by cutting its interest rates to record lows.
* An unexpected drop in euro zone inflation raises pressure on the ECB to consider fresh policy action next week to counter deflation risks and support a weak euro zone recovery that may be running out of steam.
* Concern about the outlook for emerging markets has been pressuring global equity markets for weeks. The S&P 500 is down about 2.9 percent for the month. If the broad market index closes lower, the decline would be the biggest since May 2012.
* Mattel Inc shares declined about 10 percent in premarket trading after the world's largest toy company said quarterly profit missed Wall Street's estimates.
* Wal-Mart Stores Inc shaved its outlook for the fourth quarter and full year to account for special items, including those tied to store closures and the restructuring of Sam's Club. The stock fell 2 percent in premarket trading.
* Amazon.com Inc shares plunged nearly 7 percent in premarket trading, a day after the company missed Wall Street's estimates for the crucial holiday period and cautioned investors about a possible operating loss this quarter as shipping costs climb.
* S&P 500 e-mini futures fell 17 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures lost 158 points and Nasdaq 100 futures fell 16.75 points.
* Google Inc's quarterly revenue beat Wall Street's target despite an ongoing decline in prices for its online ads and deepening losses at Motorola, the handset-making division to be sold to China's Lenovo. Google shares were up 3.2 percent in premarket trading.
* Zynga Inc shares jumped 17 percent in premarket trading. The company will shed 15 percent of its workforce to slash costs and buy mobile game developer NaturalMotion for $527 million to refresh a stalled games pipeline.
* MasterCard Inc, the world's No.2 credit and debit card company, reported a 3 percent rise in quarterly profit as more people used its cards around the world. But the stock was down 2.5 percent in premarket trading.
* In economic news, U.S. labor costs rose in the fourth quarter but there was still little sign of wage inflation amid slack in the jobs market.
* A separate report showed U.S. consumer spending rose more than expected in December, but weak income growth suggested the economy could cool a bit in the first quarter.