Some may say that class building is difficult work. I agree, but if asked to create a completely new and original kind of monster, I'm more likely to run and hide. I don't profess myself to be good at building monsters, but I do know a memorable one when I see it. I'd like to offer a single piece of advice when it comes to making a new monster...

Make it memorable!

A monster's life is short. Usually, a monster gets a single encounter to make an impression on PCs. If that impression is lackluster and easily forgotten, it's likely that the entire encounter will be so as well. To make a truly lasting impression on players, a monster has to do something that really gets the PCs blood flowing. By that, I mean that the monsters have to EARN the player's antagonistic emotions. A band of goblins, even at low levels, rarely sparks much from the imagination. Take those same goblins, give them all torches and put them down in the middle of a small village, and you've got a real problem on your hands. It goes the same way with creating new monsters.

One of my favorite monsters is the Dust Wight from Monster Manual III. It's a peculiar sort of undead because it acts like an earth elemental with power from the negative material plane. It sounds pretty cool, but what makes the monster the most memorable are the scenes you can describe with it. The Dust Wight has an ability that petrifies creatures who get within 5 feet of it. Now we're talking an encounter where a single failed save can turn into a deadly situation. On top of all this, the monster has an ability like the Rust Monster's. Any attack that hits also does damage to metal weapons or armor. I'm not an evil DM, but this monster was truly made to test the players' abilities to be resourceful and adaptable.

However, it's not just the cool abilities that the monster possesses that makes the package complete for me. In whatever description you use to bring your monsters to life for players, always consider that this foe has its own motivations and concerns. The more you can dress up a combat with blow by blow descriptions including the emotional stake of the monsters in the combat, the more the fight comes down to protagonist vs antagonist rather than just a couple of miniatures on the table.