This might be common knowledge (or so some might claim). It might also be already posted (tried to find something similar with the key words "rugged leather onyxia").
The Trash you have to kill when going to Onyxia consists of 4 dragonkin. They each have 882K hitpoints and drop 9-19 rugged leather. This sounds like a waste of time, but considering, that you
a) don't have to run around a wide area,
b) have to skin only four mobs for roughly 36 - 76 rugged leather and
c) are without competition
this might be an interesting option as a grind spot. Remember, that you can reset the instance up to 4 times, resulting in 20 mobs overall.
However, killing them might still be a bit tricky as they still hit very hard. I tried with a demo warlock and failed. Using a hunter with a tank pet worked instead. Other classes might or might not be able to solo them as I lack experience in this field.
Also note, that they sometimes drop hides, that no one really needs anymore.