Elite Monsters occasionally will spawn (especially so on Gatecrasher Difficulty Setting).
These gain titles in their names, are worth more XP, and generally have abilities that make your life worse.
For example, Spotters have high Spot/Listen, and therefore those who hide might find they are being spotted. They also see through invisibility.
Frenziers are very nasty, they hit hard, have extra attacks, and are hasted.
Defenders have extra HP and damage reduction, typically 3/-.
Hosers are typically tougher than normal spawns and have Expose Weakness.
There are more types - players are welcome to flesh out the details on this page.