Who or what are the Belua

In my current game, although they have been referred to in previous games to a lesser extent, nearly every major and minor race is at war, and being beaten by the 'Belua'. I know this as I have a constant line of Diplomatic requests from almost every other player asking for help. The Belua do not show up as a minor race either through the Dipolmatic screen or by looking at each star on the map.

So, who or what are they?