The Bog Raider is neither a living creature, nor an undead, nor a ghost. Rather, it is a ghastly mix of all of them. 

Whenever someone dies in the bogs, he is often in great fear or desperation. In many cases, these fears tie the spirit to the world of the living. Trapped between the worlds, these spirits become more and more desperate. They contaminate the environment by spreading their fear and desperation, and they bind rotten plants to the bones of their mortal remains. 

After several years, the spirit combines with its newly formed body and a new creature is born - a dreadful Bog Raider rises from the swamps. The new body consists of the skeleton that belonged to the living being the spirit used to be, wrapped round with masses of rotten plants, leaves and vines.

Still driven by fear and often completely mad, Bog Raiders walk the swamps. However, their mad and frightening wailing warns all creatures of their approach. Hungry for life, Bog Raiders attack every living being that crosses their way, vainly trying to gain their enemy's life force to satisfy their tormenting hunger. 

Bog Raiders are rather driven by madness than by intelligence which makes them to dangerous enemies. However, there are also reports of Bog Raiders who overcame their madness and actually help nearby settlements or look after their former families from a distance.

Source: Tibiawiki.com