From D&D Wiki
Pegasus Whistle- a magical item that's made of a special glass that glows silver. This item is given to a Pegasus rider (by means set by DM) to call forth his Pegasus once no matter where thow be (given the pegasus isn't dead or asleep, or locked/unable to leave the place they are). If Ryder is on another plane of ex. then the Ryder must wait one full turn for the pegasus to show up through a portal in the sky/wall or ground near the Ryder. Has only one use and can only be used by the Ryder the pegasus is bonded to. After use the whistle shatters and turns to dust.