The scholars of the medieval times, were impressed with there horsemens:
"One horseman of theirs can face 100 or 200 horsemen of the infidels."
Abu al-Hassan Ali ibn Hussein ibn Ali al-Massoudi, X century AD
Of course this cannot be proof for there ability, but i bet the battles won against the strongest medieval nation in 7 century the Byzantine empire, are enough proof for there ability, also the crushing victory toward the arabs surrounding Constantinople during 717-718
As Michael the Syrian writes:
"The Arabs were attacked by land both by the people from the city [Constantinople] and by the Bulgars, and in the sea - by the Roman ships, and on the other side of the sea [on the coast of Asia Minor] by the Roman vanguard. They couldn't get out of the camp to a distance greater than two miles, while they were forced to search for wheat. The Bulgars attacked the Arabs and slew them; those latter [the Arabs] feared the Bulgars more than they feared the besieged Romans."
Later in the years, due to the mountain region of the Balkans, bulgarians were forced to use infantry, and because the slavic tribes, rarely used horses, we were a perfect mix, bulgars were the cavalry, and the slavics were the infantry.
Bulgarians also as the mongols lived on there saddle, being nomadic people comming from the Steppes. Not only that but bulgarians worshiped the horse - http://nolimit.cult.bg/presscenter/i...ski_konnik.jpg
And the tail from the horse was like a flag for the bulgars, it was used on there spears, as a symbol - http://antika.avonet.cz/upload.cs/b/...b_1_scyth2.jpg
There was strong laws for the treatment of the horses, riding a military horse in a peaceful time resulted in death, not giving enough food to your personal horse, resulted in death, in fact every mistreatment of this holy animal, resulted in death.