Bukkit

1.flying squides   | a must !!!
2.shotbow skeletons
3.flying heads
4.slimes,with items(entities) in them,they pick up everything,and you get it when you kill them
5.aggressive cats
6.underwater ghasts
7.ghost boats
8.aggressive iron golem
9.aggressive snow golems (spawn in groups)

Some of them may not be possible, but who knows? We'll find out when we test it out.

Some of them may not be possible, but who knows? We'll find out when we test it out.

Some of them may not be possible, but who knows? We'll find out when we test it out.

Some of them may not be possible, but who knows? We'll find out when we test it out.

Some of them may not be possible, but who knows? We'll find out when we test it out.

To reach the flying effect, they can ride on a bat ;)

pretty sure ,that everything of this is possible. only ones that could cause some little problems are 4 displaying entities inside entities could be hard... and 6 the water could probably cause some problems to the fireballs...