Medium Rare

Medium Rare is an exploration achievement earned for killing one of the rare mobs found all over Outland.

Blade's Edge Mountains

 * Hemathion - Vortex Pinnacle.
 * Morcrush - on or around Razor Ridge, or in front of Gruul's Lair.
 * Speaker Mar'grom - ogre camps in the low valley down the center of the zone.

Hellfire Peninsula

 * Fulgorge - crevices south of Honor Hold heading west; between the Great Fissure and Hellfire Citadel; between the Great Fissure and Temple of Telhamat, west of Falcon Watch.
 * Mekthorg the Wild - on or around the ramparts of Hellfire Citadel.
 * Vorakem Doomspeaker - in the Legion Front, the Pools of Aggonar, or north of Thrallmar.

Nagrand

 * Goretooth - in the lakes.
 * Voidhunter Yar - around Oshu'gun.

Netherstorm

 * Chief Engineer Lorthander - any Manaforge in Netherstorm except Manaforge Ultris.
 * Ever-Core the Punisher - around Manaforges Ara, B'naar, or Ultris.
 * Nuramoc - flying on the south edge of the island with Area 52, north of Manaforge Duro or west of Forge Base Oblivion. Nuramoc is a highly prized pet for hunters, making it additionally hard to find and kill.

Shadowmoon Valley

 * Ambassador Jerrikar - Coilskar Point, Illidari Point, Eclipse Point, Ruins of Baa'ri or Dragonmaw Fortress.
 * Collidus the Warp-Watcher - various locations, concentrated around the Black Temple. Primarily on the northeastern and southwestern edges of Shadowmoon Valley, as well as west of the Hand of Gul'dan.
 * Kraator - in and around lava.

Terokkar Forest

 * Crippler - around Auchindoun.
 * Doomsayer Jurim - patrols the main roads in the forest between Hellfire Peninsula, Shattrath and the Bone Wastes.
 * Okrek - can spawn in any of the Arakkoa encampments, except for Skettis, the encampment near Bleeding Hollow Ruins, and the Arakkoa ghost area in the Bone Wastes.

Zangarmarsh

 * Bog Lurker - around the edges of Broken villages.
 * Coilfang Emissary - around the naga pumping stations.
 * Marticar - along the north, east, or west map borders, in the water.

Macro
A handy macro for targeting them, as you fly about: /tar Fulgor /tar Mekt /tar Vorak /tar Martic /tar Bog Lurk /tar Coilfang Em /tar Crip /tar Doomsay /tar Okrek /tar Goretoo /tar Voidhunt /tar Morc /tar Hema /tar Speak /tar Nura /tar Ever-Core /tar Chief Engineer L /tar Collid /tar Kraat /tar Ambassador J

Tips
It's advised to use a rare finding addon, such as NPCScan or SilverDragon, when going for the achievement to make things quite easier when flying through the zones.