A gragol is a Ferengi wish spirit, similar to a Terran genie.
In 2370, Nog told Talis Cedra a Ferengi story about two brothers. The younger brother had a gragol, with only one wish, in a bottle. He convinced his older brother to buy it. When the older brother opened the bottle to make the wish, he forgot one thing: that he stammered when he got excited. By the time he finished his wish, the gragol had flown away. So the older brother broke the bottle over his brother's head and told everyone that he was an incompetent swindler. (DS9 novel: Warchild)