Have you noticed that some mobs and beasts have a border around their icon? These are rare mobs.
In fact, according to WoWWiki there are two kinds of rare mobs: The rare and normal mobs, and the rare elite mobs. There’s a slight difference in the border. The rare mob has a silver dragon without a wing, whereas the rare elite has a silver dragon with a wing.
These mobs have better loot than other mobs, and the first, you find in Northrend, even give you an achievement. So it’s always handy to find the rare mobs, and there are means to help you with that.
You have to use an addon, if you want to be alerted, when a rare mob is near you.
At the moment, I use two addons (well!). The first is called SilverDragon, and it works everywhere in Azeroth and Northrend. The second works best in Northrend at the moment, but it is more detailed. It shows an icon above the head of the mob. That addon is called _NPCScan.
Good mob hunting!