As their name implies, the Giant Deer are depicted as normal deer, but of an abnormal size.
Two of them are briefly seen in their cube cell above the Gorilla's cube during the elevator scene.
Tampering with the deer antlers summons them.
They are mostly a real world fear. They may have been inspired by the scene in The Ring 2 where a herd of deer viciously attack the main character's car or may have been inspired by legends of early mans encounter with the Irish Elk , a massive deer that lived during the last Ice Age.