Hellhounds are a myth from ancient folklore and
are symbols created and limited only by your imagination. They can not be summoned but they can be read about from works of fiction and old legend.
From Wikipedia: “A hellhound is a supernatural dog in folklore. A wide variety of ominous or hellish supernatural dogs occur in mythologies around the world. Features that have been attributed to hellhounds include mangled black fur, glowing red eyes, super strength and speed, ghostly or phantom characteristics, and a foul odor.”