Bukkit

Config
Allow spawn for example allowWraithSpawn:true
Health for example wraithHealthLimit:10
Damage for example wraithDamage:5
Drops for example wraithDrop:276
Speed for example wraithSpeed:5
Mobs
Killer
A zombie with ugly and scary face,spawns silverfishes when at low health,40 HP,has a iron sword and it's speed is 0.3
HighLeaper
A zombie with no damage from falling and jump boost 5.
Speedy
Silverfish with very low health and damage but it is very fast
Attacking Squid
It's a aggresive squid! Has everything that does have squids but weakly attacks.
Spitter
A low health zombie which when hits you gives you blindness for 15 seconds. Also it is weak. Stuns for 3 seconds.
Death Skeleton
Neutral,but turns into aggresive when you will get close to it for 4 blocks.While neutral it doesn't move.When turned into aggresive then it plays a ghast hurt sound. It also plays ghast sounds when dies,hurts.When aggresive it is fast and very strong. In first hit makes the target slow,weak,poisoned,wobbles the targets screen,deals 8 damage on hit,hunger and invisibility on targets hit. Has 65 HP. When near to witch in 50 blocks the 13 disk plays.A only way to get through the death skeleton is sneaking. If you get close to the death skeleton by 4 blocks when you are sneaking,then the death skeleton will be still neutral.
Tank Zombie
150 HP.Speed is 0.7. Hit deals 5 damage and knocks back you like with knockback 3/4 enchantment.On death it plays ender dragon death sound.It also growls like ender dragon.
Charger
A zombie which when charges it will have strongness 3 swiftness 3 regerenation 1 and when hits a player then the player gets like stunned like he just is sleeping (not really the animation of sleeping) but he is not in the bed he will not sleep. The player will be stunned.
Jockey
A spider that when hits the player,he will ride the player. The jockey will control the player. When the player gets rided by jockey then the third-person turns on.
Wandering Death Skeleton
If the death skeleton will survive the night and the daylight will come he will not burn,but he will wander. Just walks around. Has everything same that haves death skeleton.
Horseless Horseman
A skeleton with some strong armor,wearing jack-o-lantern on his head,a strong stone axe holding his hands,riding a spider (lol maybe not) or skeleton horse.
Horse Jockey
Just a zombie riding a zombie horse or skeleton riding a skeleton horse.

I got the death skeleton idea from one game called Left 4 Dead.It called Witch. You can see it in http://left4dead.wikia.com/wiki/The_Witch
Also the same with the Tank Zombie.

Most of these are too specific and unnecessarily complicated, or are just a slight variation from the vanilla mob, making it a trivial addition that can be accomplished by any other plugin and thus unneeded. Adding some of these would probably only clutter the configuration more...

Perhaps what would be more in order, is the ability to "design your own mob", so that you can make some of these mobs (the ones that involve just slight variations like potion effects or additional health) at your own whim without forcing clutter on other people. Consider using the OtherDrops plugin:
http://dev.bukkit.org/bukkit-plugins/otherdrops/

The problem with the super-specific ones is that you want them, but 99% of other people might not.

These are just my thoughts; it's up to the developer to decide.