Golden Sabercat

There used to be a sabre mount "parked" outside of gadgetzan, it was unique due to its skin: exactly the same as any typical tiger found in STV, with the addition of a saddle.

for unknown reasons the skin of the stabeled tiger was changed into a "standard" skin - if I recall correctly this change took place roughly half a year before TBC.

Sounds to me like this could be that specific tiger :)

You are somehow correct. You can see it from the spell ID, 16060. It's a low number, and spells in that range are spells that were available as far back as alpha-release of the original game. So, it's something thought of far back, but never implemented. We don't even know if there exists an item for this mount, though it's likely it exists.

This model would also work quite well as a hunter pet as well as the rest of the unused models.

http://www.wow-petopia.com/family/cat.html