Whistles of Summoning (usually coupled with a chain for wearing about the neck) were wrought by the platypi and able to summon the creature of which image was etched upon it. For example, the whistle able to summon a pterodactyl had a picture of a pterodactyl upon it.

In 966 GUE, Whistles of Summoning (of the pterodactyl variety) sold for Zm20.

SOURCE(S): Wishbringer, Wishbringer (novel), Beyond Zork