The giant is said to be very tall and can normally be found around the forest area of the mountain (where the player first goes on thief first mission in Whetstone). The giant is said to be hostile, however not many players have seen the giant, and the ones who have were only able to by the use of a mod.
In real life there is a similar myth in Nordic mythology, namely the Troll. The Troll is a giant (commonly shown as a creature made from stone) which is believed to live in forests, and is believed to be a nature God.
Sightings specific trees from a considerable distance, may make one of the trees look a gigantic figure, easily giving an impression of a Giant and may possibly be the origin of the myth.