Obviously, a semi-centaur would have half the number of limbs of a centaur, ergo, this is a creature with three limbs (2 legs and one arm, perhaps?). A creature with two horse legs and two human arms would be, presumably, a dicho-trito-centaur (two thirds of a centaur).
*Hoping Qroqqa or Milo will correct my preposterous Greek prefixes*
"Ever wished you were a 7-foot tall semi-centaur with the body of a human and a horse's legs? Er, me neither. But there's plenty of people out there who do, and Seattle-based fantasy artist Kim Graham has come up with a killer Halloween costume that makes you 14 inches taller and gives you uncanny-looking equine legs and spring-loaded cloven hooves."