Heliopaths are spirits of fire — huge flaming creatures that gallop across the ground burning everything in their path. They may be purely creatures of legend or folklore, but Luna believed during Harry’s fifth year that then-Minister for Magic Cornelius Fudge had an army of Heliopaths at his command (OP17).
Luna Lovegood tells the Dumbledore's Army group that Cornelius Fudge has an army of heliopaths, creatures that she later describes as "spirits of fire" to Neville. She says this with "her protuberant eyes widening so that she looked madder than ever." She continues by saying that they are, "great tall flaming creatures that gallop across the ground burning everything in front of -" but is caught off by Hermione, who tells Neville that they don't exist (OP17).