A Necromancer is an evil wizard who practices in the necromancy. Necromancy is the power over the world of the dead. Their powers enable them to raise corpses from the ground to create armies of Skeletons and Zombies. Necromancers steal the life-force from other living creatures to extent their own and to increase their strength and toughness. Often necromancers are from human stock but this does not rule out other races from practising necromancy.

Nightmares are undead horses they are one of the greatest aids to the undead. These are the bodies of great war-horses that are brought back to the unlife by necromantic powers. Nightmares unlike mortal horses have the ability to move through terrain that might be blocking there path they can move though walls and dense vegetation with ease

I will accept the following races for Necromancers as follows....

MOST NECROMANCERS
Humans
Elves
Basilisk

RARE NECROMANCERS
Arions
Griffondyles

Arions and Griffondyles are rare as being necromancers due to their lack of natural magic...they must be specifically trained and apprenticed in such arts.

EVEN MORE RARE NECROMANCERS
Centaurs

Centaurs don't usually if ever master magical arts. And are not gifted such abilities naturally...so yeah....need good explanation to get one of these as a necromancer.