Plugin for spigot 1.8.8, when a player joins the server, it is transformed into a random hostile mob, either a blaze, creeper, pigman, zombie, enderman, skeleton, zombie baby, spider, slime, witch, ghast, cave spider, silverfish, magma cube.

@EventHandler
public void onJoin(PlayerJoinEvent e) {
    EntityType random = EntityType.values()[new Random().nextInt(EntityType.values().length)];
    if(random.isAlive()) {
        Entity entity = e.getPlayer().getWorld().spawnEntity(e.getPlayer().getLocation(), random);
        e.getPlayer().teleport(entity.getLocation());
        entity.remove();
    }
}