In the stories of "The Lord of the Rings," Tolkien made some pretty gross enemies. They consists of blood thirsty orcs, massive trolls, wargs (mainly in "The Silmarillion"), and even more wicked things like the Balrogs. All these things come from Melkor/Morgoth, a Lucifer figure in the story. He was given power and this made him prideful, so he wanted to destroy all that he did not make. So he makes the evil of Middle Earth.
Orcs. Not at all a pleasant sight. Their skin is overrun with filth, boils, and blood. For all orcs hardly ever agree. Yes, they will stand together to fight all that is good in this world, but when they aren't killing anyone, they usually fight amongst themselves. When this happens all the orcs, except maybe two or three, are killed. This made things very easy for Sam, when he had to invade the tower in Mordor to find Frodo. All the orcs had killed themselves already!
Another little nasty are Trolls. Not exactly little. They are huge twisted evil things that use their size to crush, pull, and destroy all around them. The most famous of these is the Cave Troll in Moria, where it took the whole Fellowship to bring it down. Another famous Troll is the one that Pippin killed at the battle of the Black Gate. This was an amazing victory, because a four foot hobbit was able to defeat a ten foot (or more) Troll! It seems like David and Goliath!
The most menacing of all of these are the Balrogs, one of the greatest servants of Morgoth. For the Balrogs were once Maiar (the servents of the valar) and are most powerful compared to the other things that Morgoth manipulated for his use.They are both shadow and flame; no mortal nor elf can defeat them. Yet a wizard with the grace of Iluvatar (chief of the valar, AKA the one and only God) was able to defeat one. Gandalf defeated the Balrog that was awakened in Moria which was a great feat, for anyone.
All these things are pure evil. They are the devices of Morgoth who, above all things, hates the valar. The valar love the beauty of Middle Earth and men, elves, and dwarves. Therefore Morgoth hates them. He spun all these things out of his head in order to destroy what is good in this world. Yet after the mightiest of all his servants, Sauron, was destroyed, the evil was scattered and leaderless. And the days to come were very peaceful. All ended well.