According to Eskimo lore, an Angiak is a child of the living dead. Eskimo often had to give up their new-born children during harsh times and would do so by abandoning them in the snow. Unless the Eskimo tribe moved to a new land, it was believed that the ghosts of these babies would come back to haunt them. An Angiak is said to gain strength upon each visit to the tribe, until it is powerful enough to seek revenge on the elders.
When Sam tells Dean about his vision including a "dark shape" stalking Roger Miller. Due to finding nothing at Roger's apartment, Dean questions Sam if whatever they are hunting isn't connected to the Miller house, but assures him it is connected to the family, questioning Dean if they could be dealing with a vengeful spirit. Dean agrees it could be a possibility as there are things that are known to latch onto families and follow them for years. With Sam mentioning angiak and banshees as examples.
Dean: Well yeah, there's a few that have been known to latch onto families, follow them for years.
Sam: Angiak. Banshees.
Angiaks in Lore
Centuries ago, the Inuit people of Alaska would, in desperate times, be forced to take children that could not be cared for and abandon them out on the frozen plains. After a child died, its vengeful spirit would sometimes return and animate the body, creating a vampiric revenant known as an angiak. At night it would steal back into its family’s home and nurse from its mother’s breast. When it grew strong enough, the angiak would develop the ability to shape-shift into various wild animals, which it would use to kill off its family members one by one.