Hmm, thinking about it, it could be true. So Hippogriffs might be like Liger & Tigon or Mule ( interestingly enough there is a bird hybrid also called that ) & Hinny. The cases in Mythology are male Griffon + mare, so maybe in case of Stratus it is female Griffon + stallion. At least he is better than all those fanmade ones that give them straight pony heads and which often have mismatched limbs ( i.e. left hand a claw, but right hand a hoof )