You can earn from 300 and up gold in a matter of minutes with this rare pet.
It works only if you are horde and you can farm this pet from level 20’ish in fact, so no need to have a real high level. In fact, even if you are Alliance, you can very quickly level up a horde to 20 or 25 and let it camp there and get the pet.
“There” is refering to Ambermill in Silverpine forest (see the red circle on the map). It is a starter area, so no risk for you to be ganked while questing and since the mobs you have to kill are Dalaran-mobs, they are friendly to Alliance. This is why they can’t farm that pet (and this is also why it is worth more gold in Alliance’s than in horde’s AH).
In Ambermill, behind the fences, there are a big building and two houses. When you are in front of the big building, the treehouses are on your left and on your right.
The mob that may or may not have the cat is called “Dalaran Spellscribe“, it is one of the rare mobs I know that can be male or female.
You can write /target Dalaran Spellscribe to see if he or she is there, but you can also, especially if you are high level because it goes quick, just clean the place of all mobs. There is always at least one green item, wool cloth, linen cloth and a chest that may contain silver bar, gems, etc. Just by selling what I got from there while making the video, I made around 80 gold.
Now, when the Spellscribe is there, you kill him or her and loot. The Spellscribe is generally either in the big buidling or in one of the two shaks. You can see the video for more details.
I have generally been lucky and the Black Tabby has been there every second or third kill, so it is a pretty easy farmed pet.
You can sell it on neutral AH or, with help of a friend, on Alliance AH for extra gold.
At the moment, I noticed that prices of pets have dropped on all the EU-realms I am on (6 realms from RP to PvP realm and Alliance or Horde).
But I think it would be a good idea too, to get a 5 or 6 of these cats and wait for after Cataclysm to sell them, not sure they will be farmable then.
Beside, lot of players have stopped playing and are going back “after Cata”, there might be a greater need for pets.
If the pet is still farmable after Cataclysm, no problems, it is a minor investment and you can always sell them anyway.