Does a Dullahan's CR account for his Heavy War Horse already? Or should that be factored in to the XP/general challenge of the encounter?
http://www.d20pfsrd.com/bestiary/monster-listings/undead/dullahan
The ability to summon the mount is one of his features so the CR should already be included.
Edit: Also if you read to the bottom of their description you will see that the ability to summon a more powerful horse raises the dullahan's CR so yes it is included.
The horse is a feature of the monster (Summon Mount (Su)). It's already factored in.
Thanks
As is the ability factored in to raise shadow mastiffs if it kills folks.