The creature, in the form of a crystal, was one of a number of artifacts that Vash brought back through the wormhole and subsequently attempted to sell at auction in Quark's. Vash was apparently unaware that the crystal was a lifeform. The Deep Space 9 assay office clerk thought that the crystal was Promethean quartz, but Vash pointed out that the molecular density and refraction index were much higher. At the auction, the crystal received bids in excess of one thousand bars of gold-pressed latinum, before Q started to make a mockery of the event and bid ridiculous sums up to one million bars.
Unbeknownst to the crew, the presence of the crystal on the station was the cause of an unexplained series of massive power drains, and the source of a graviton field that was pulling the station toward the Bajoran wormhole. Before the auction could end it was interrupted by Commander Sisko and the senior staff, who had discovered that the crystal was the source of the problems. At Sisko's command, the crystal was beamed 500 meters off the docking ring, where it metamorphosed into a spaceborne lifeform. The lifeform flew into the wormhole and, presumably, back into the Gamma Quadrant towards its home. (DS9: "Q-Less")
Background information Edit
The creature was referred to as "the embryonic lifeform" by Sisko in his closing log narration. The word "embryonic" was not capitalized in the script, so was likely intended merely as a description rather than a name. The script directed that the optical shot depicting the spaceborne form of the creature should show a "winged energy creature".