An Old tooth can be found while digging at the digsite using a trowel (obtained from the Varrock Museum or an Examiner, do not use a gardening trowel) on the digging area west of the animal skull or panning in the panning area with a panning tray. The odds of digging up an old tooth are around 1/50. The archaeological expert says, "Oh, an old tooth. It looks like it has come from a mighty being. Pity there are no tooth fairies around here!"
You can trade this item with the Tooth Fairy in combination with other teeth, for a new pet, after A Fairy Tale Part III - Battle at Orks Rift, or sell it to her for 2000 coins
A tooth can be found here, at the very northeast corner of the Digsite.