9) Prinny
Reasons: Prinnies are so adorable. Prinnies were once humans and became the way they are after they have died. As a prinny these sinners must pay off their sins (though depending on whether they work in the multitudes of Netherworlds or Celestia, they would be overworked and underpaid or simply do house chores respectively). Don't underestimate these little guys because they know how to utilize machetes and bombs. In some Netherworlds, prinnies have actually even learned how to use a weaker version of Pringer X's signature Pringer Laser. Also, if all else fails just chuck em like they were a grenade because they explode if they're thrown. If I had to talk about specific prinnies such as Kurtis, Fuka, or the Hero Prinny this section would go on forever. Everything is better with penguins.
8) Alraune
Reasons: Alraunes are not as cute as prinnies but they make up for that with other attributes. They can use their plant bodies for a variety of different attacks such as buzz-saw flowers, thorn vine whips, and even growing entire forests. They also have an many helpful passive abilities which allows her to heal nearby allies, cure them of ailments such as poison and paralysis, and lessen the amount of effort allies need to use powerful skills. This is one thorn that you don't want on your side.
7) Wood Golem
Reasons: The polar opposite to an Alraune, wood golems are all about dishing out the hurt. Not only can they deliver major damage they can take it due to their high durability and self-regenerative ability. On top of their regeneration they can also gain stamina back equivalent to 10% of the damage they incur on to a foe. If you wanna fight the embodiment of the land's anger, you'll be fighting for a good while unless you set it on fire.
6) Fire Demon
Reasons: With physical strength that rivals that of the wood golems and the ability to project fire from it's body, it is easily one of the deadliest species of demons of the NIS universe. When it uses melee attacks it can drain the energy of it's foes to increase his own energy supply to allow him to unleash more fire on the battlefield. The only real problem with him is...water. Whatever the case, may the fire the of the Netherworld incinerate my enemies.
5) Nekomatas
Reasons: When compared to wood golems and Fire Demons, Nekomatas might not be up to par in physical strength but they make for that with their martial arts skill and distracting sexiness. Not only do they have a plethora of martial arts techniques but they can also unleash kamehameha blasts, though being as species of cat-like creatures they are easily distracted by things such as butterflies and crinkling paper.
4) Zombie Dragons
Reasons: Oh boy. Combining the unusual physiology of zombies and the terrifying powers of a dragon, they are anything but an easy opponent. These guys can take even more punishment than wood golems, breath dark fire, and have a set of chompers that put Arlong from One Piece to shame. If a movie ever comes out about zombies in the medieval Europe, it needs zombie dragons to make it complete.
3) Succubus
Reasons: I think you can see two of them right off the bat. Sexiness aside, Succubus are more magically inclined than most of the other monster units on this list, using high level fire spells, healing spells, spells that transform her foes, and a magical spear. On top of their own plethora of magical abilities, they get along well with Nekomatas whose physical capabilities compliment their own magical talents. It is implied that Succubi are the descendants of fallen angels due to the fact that they can use healing spells and have wings (In the NIS universe, Angels have a monopoly on healing magic and demons in which can use them have some kind of connection or relation to angels).
2) Mystic Beasts
Reasons: First, that wicked horn. Second, it's a wolf and I like wolves. That aside, these creatures can move as fast as the wind and can even utilize it's power.
1) Reapers
Reasons: I don't even know why I have to go into depth on this one, but I will anyways. These creatures are embodiments of death in which help traffic souls on their way to becoming prinnies and reincarnation from prinnidom (is that a word they use in any NIS game?). His scythe is awesome and is used to butcher his foes and throw it like a boomerang. If his scythe isn't enough, he can send you into a dimension of terror and fear which destroys your body from the inside and out. I gotta wonder though, what's underneath that skull mask?