You choose which creature you control to exile (if any) as the enters-the-battlefield ability resolves. Once you make that choice, no player may take actions until the chosen creature is exiled or Wormfang Drake is sacrificed and the ability has finished resolving.

If you don’t control any other creatures as the enters-the-battlefield ability resolves, you must sacrifice Wormfang Drake.

If Wormfang Drake leaves the battlefield before its enters-the-battlefield ability resolves, the “leaves-the-battlefield” ability will trigger and resolve first. It won’t return anything. Then the enters-the-battlefield ability will resolve. If you exile a creature you control, it will be exiled indefinitely. Wormfang Drake won’t return it.