Spiky ? Block
From the Super Mario Wiki
The Spiky ? Block is a type of block first found in New Super Mario Bros. This block is similar to a regular ? Block; however, it has spikes at the top. It also spins around, requiring the player to mind their timing to avoid getting hurt. Hitting or Ground Pounding the nonspiked side will turn the block into a normal Block.
The blocks only appear in World 6-3, where four of them are seen, three of which contain power-ups.