Originated from Irish- Celtic mythology, the Fomorians are described as a people having a goat head, yet having the body of a man. Some texts would include that the Fomorians would be have an eye, an arm, or a leg in the middle of their body, for example Cyclopes. Yet these same texts would mention that some indeed were beautiful.
The name meaning is often debated. The names ranged from, Fomoire, Fomoiri, Fomorians, Fomors, Fomori, and Fomóraiġ. Medieval Irish scholars assumed the name contained the word muire, which meant "sea". In 1888, John Rhys suggested that it is an Old Irish word composed of fo which meant "under/below" and muire "sea", concluding which would conclude that the combination would mean under the sea. Seeing the word fomra, Kuno Meyer explained that it refered to land by the sea, rather than under. Whitley Stokes and Rudolf Thurneysen, decided to connect the second element *mor with a supposed Old English cognate of mara which means to this day night-mare. Marie-Louise Sjoestedt interprets the combination of fo and mor as meaning inferior, or latent demons. some even have just called them goat- headed giants. Maybe they should be called goat-headed sea giants.
Some sources claim that the fomori uses the element of water to create fog, storms, (including ice storms), and bring about diseases. They could even do sorcery. Some consider them as a group of hunters, though I have found no information nor anymore sources to support such a conclusion. They were deemed dangerous and eventually were exiled.
They are exiled fae that were originally merpeople but had managed to adapt to land. So they changed from the element of water to nature, which is another reason they are dangerous. They (according to research) consists of the elements of darkness, water, and earth. Thus the reason they can control and use magics such as disease and storms, fog, ect. They were exiled by the unseelie court (which is really bad considering that the seelie court are the ones that usually exile.) The question is why and who had the power and enforcement to put them in the void?

http://www.pantheon.org/articles/f/fomorians.html
http://en.wikipedia.org/wiki/Fomorians
http://www.shee-eire.com/Magic&Mythology/races/Formorians/Page1.htm
http://forgottenrealms.wikia.com/wiki/Fomorian