Yup so for you personally Each Rag Kill Quest will have a 25% chance at a mount. And that mount will be 50% 12b and 50% Blazing Drake.
Ya, sometimes your just unlucky. I can’t tell you how many times I’ve ran dungeons in Classic where the drop chance for the item is 30% and it took me 15 runs to see it drop. Sometimes RNG just says screw you in particular. (Like my ret pally who got that 2-hand agility sword from the Cache at 1/6 hero which for some reason is still on the loot table for Paly/DK/War)
But on average most people will see a mount every 4-6 quests. It’s best to look for skip groups so you can kill the boss in 4 mins, open chest, hop on and alt and do it again.