Well i think a cool mob would be a snake, for multiple reasons.One reason would be so it can fit in with the new jungle biome, coming out in 1.2. This would make the jungle way more realistic. Then the mob would be uniqe because it would be able to climb vines, and it would be camoflauged. Now, if a player kills a snake they will get a poision sac. A poision sack will be used to make poison weapons, and potions. Snakes would also be a hostile mob that spawns day and night making the jungle dangerous. They would also be able to poison a player and would look really cool.

Please tell me what you think,
Karatyman,