An enraged enemy is not a different type of enemy in the game, it's simply a regular enemy that has become enraged. To enrage an enemy, you must throw a few grenades at him with such power that he moans and frowns. Eventually a screaming sound will play and the enemy turns red. This indicates that he is raging. I've included a video by the user ONLINEgames03 on YouTube that shows how to enrage an enemy.
I found doing this on level 14 on Desert Strike to be the easiest spot. There are three enemies here, conveniently lined up one after another.
Launch, at full speed, grenades at them and either hit them at such an angle the 'nades briefly hits them and continues off-screen, or use the Abort button to cancel the grenade. With the unlimited grenade reward given for completing Planet X, you can easily enrage all 3 of them and kill them, then press Repeat and do it all over again.
If you also have the detonator reward from completing Lost City, this is also the perfect spot to get Triple Enraged Frag.
This is also a very good spot to grind the 1000 grenade kills.