for number 3 I would say have it's third eye shoot a projectile that slows you (like the spell) so it's harder to get away from them (I assume they run fast like actual centaurs.)
Linkified: oni-taur
Per the SRD, Oni take on humanoid form due to association with humanoid evils. So this Oni must spring forth in a land with centaurs.
Likewise, their ability to shapechange to any humanoid, might translate to centaurs and similar creatures. Maybe even horses.
For powers, an aura of pain or doom might be good. Enhanced trample or charging might be good as well.
/cevah