|Chromehound and Chrominius (with magical pet biscuit)|
The most notable rare in this zone is without a doubt Chromehound, an eye-catching level 49 core hound that shares the exact same color and model as the raid boss Chromaggus in Blackwing Lair. Yes, those of us that have tamed Chromaggus already - hes not unique anymore. Quite honestly I don't think its a big deal though. This skin is still rare enough as it can only be tamed via these two sources. If anything, I think obtaining Chromehound as a pet can prove to be even more difficult than Chromaggus due to competition from others (especially with crz) and his spawn timer which seems to be somewhere in the 3 hour + area.
The second new rare added to Blackrock Mountain in 5.1 is Crystalfang, a level 49 red shale spider. Crystalfang is one of only two tamable shale spiders that can be found in this color before encountering the level 80+ shale spiders in Deepholm and The Stonecore. That being said, you will still need to be at least level 69 and in beast mastery spec to tame him.
His spawn timer is similar to that of Chromehound, and you can find him in the lowest level of the mountain in The Masonary just west of the entrance to Blackrock Depths. He spawns right in the corner, surrounded by three aggressive dwarves. I have marked his spawn point in red on the map below.
The final rare in this zone has been a long time inhabitant, and can really only be called a rare thanks to his lovely silver dragon portrait. His spawn timer is incredibly fast - I'd say somewhere around 10 minutes give or take. This guy always drops the Vilerend Slicer, and like other rares - a very decent amount of experience for leveling characters. You'll find The Behemoth patrolling back and forth east to west through The Grinding Quarry on your way to Blackrock Depths.