Easiest way to increase your reputation standing with the Sporeggar is to farm and return Sanguine Hibiscuses.
To do this, the most efficient place to go is Underbog. Sanguine Hibiscuses both grow there and drop from the mobs. I usually just go round the beginning killing all the mobs, then partially skip my way past the nagas to get to Ghaz'an. The tunnel after him (her?) usually has a few more hibiscuses, so it's worth the jog. Down with the last boss, out of the instance and reset. You can do an instance 5 times per hour without triggering the instance lock (you know, "you have entered too many instances recently.").
I know some people rather do the other repeatable quest of killing nagas, but I find the Hibiscuses to be more efficient.
For most, the primary reason to farm this rep would be the Tiny Sporebat companion pet they sell at Exalted.
This item is currently unobtainable on Live servers, because it was a reward for the first person to complete the quest Bang the Gong! on each server. Currently, each new server to open will already have the Gates of Ahn'Qiraj open and the questline to open the gates was removed from the game in Cataclysm.
I have personally never seen this mount, and I even have no idea who is the Scarab Lord on my server. However, there undoubtedly is no rarer mount than this.