Erm... Well no you dont get the creature they where carrying. It depends whos side the creature was on when it was killed.
You see sac docs stuff the soul back into the creature and heal them to 1/2 health, so if you kill a sac doc with an enmey creature with it then the enemy creature will be reanimated with 1/2 health, it will then attack you if your too close.
This is actually very painful ..esp with lvl 9 creatures