use  /playas [mobname]  to start.
use  /playas stop       to end.

Available creatures:

CREEPER
BAT
BLAZE
CAVESPIDER
CHICKEN
COW
ENDERMAN
GHAST
GIANT
HORSE
IRONGOLEM
MAGMACUBE
MUSHROOMCOW
OCELOT
PIG
PIGZOMBIE
SHEEP
SILVERFISH
SKELETON
SLIME
SNOWMAN
SPIDER
SQUID
VILLAGER
WITCH
WITHER
WOLF
ZOMBIE