Finally got the red infernal on my 250th kill (I didn't have Rarity on my earlier kills, hence the incorrect number in the screenshot).
Still cannot believe how it happened:
I had created my third "Eli + Gul'dan only" group for today, but no one signed up. After ten minutes or so a priest finally appeared in the LFG tool. He asked me to invite his mage friend, which I did, too.
Fast forward to Gul'dan, and I see the [Fiendish Hellfire Core] drop for the mage. I'm disappointed, but congratulate him in chat. He replies: "you want it?". I'm excited and ready to roll, when the priest says that he already has the mount. Boom, trade window opens and I get the mount. I'm so happy right now
The best part: the Mage actually had none of the infernal mounts, but he didn't want the red one, and was kind enough to give it away. I decided to create a char on his server just to buy a WoW token and send him the money as a "thank you".
Here's a combo picture with the hunter from my second account: