Reptilians
A genetics akin to reptiles, these are highly advanced entities but viewed as being of a negative, hostile or dangerous disposition since they regard humans as a totally inferior race.  They would 
perceive us much the way we would perceive a herd of cattle.  They are carnivorous in regard to humans.  There is supposedly a "driven" planetoid or asteroid inhabited by 30 million of these lizard-folk that is to enter our solar system in the mid 90's if the present schedule is kept.
 
They consider earth to be their own ancient outpost and would expect to have complete control of the entire planet upon their return. Their own planet is becoming unable to adequately support life and they need somewhere else to live.  These are the aliens who are served by the type A greys.