Just done a bit more reading, and I cant say I can answer your question... in fact I can say Im more confused! According to Bells thesis, the seven brothers are also known as the seven riders. They arose when Tsiu Marpo was born...
Due to his great maliciousness and hatred, six might demons emanated from Tsiu Marpo’s body. From his head the black obstructive might demon arose. From his white bones the divine might demon arose. From his body heat and radiance the rock might demon arose. From his blood the defiling might demon arose. From his pus the serpentine might demon arose. From his rotten garments of flesh the knife might demon arose. Having become associated with 30 red mountains and plains, these seven deities slaughtered everyone in the vicinity. They consumed the life-energy of all sentient beings and brought ruin to the three realms. 44
So if they arose as a result of Tsiu Marpos hatred and maliciousness, what confuses me is this:
Later, in India, the great spiritual master Padmasambhava arrived at a charnel ground on the banks of a boiling lake of man-eating demons. At the midnight hour of the day he arrived, seven wolves with blood-clotted hair cried out. In response, the spiritual master manifested the form of the glorious Tamdrin (Skt. Hayagr?va). The wolves retreated to their masters, the seven horsemen, and the latter prostrated themselves before Padmasambhava.
I thought Karche Marpo was an emanation of Hayagriva?
So did Hayagriva manifest as one of Tsiu Marpos riders (Karche Marpo), then as himself (Hayagriva) addressing Karche Marpo as one of the riders?