I've heard of the scopas tang (zebrasoma scopas) as well as one of the mimic tangs referred to as "chocolate". Mimic tangs are cool as heck.. scopas tangs have the same body type as yellows, but aren't the flashy neon color.. and aren't any cheaper.. so that is why most people don't go for them.
The link will show you a picture of an adult. A juvenile can be seen on pg. 145 of Scott Michael's Reef Fishes Vol. 1. Like JeremyR said, the juveniles mimic two different species of centropyge angels, depending on which species of angels are in the area.
I assume one of the reasons they aren't all that popular is because they reach nearly 12" when fully grown. Given the activity of most surgeonfish, I imagine you would need a pretty large (and long) tank for one of these to truly thrive.
They are very cool but do get big quickly. An 2 inches all yellow tang grew to the adult coloration and about 6 inches in 1 year in my tank. He is a very good tang, not mean at all. He is the biggest of the 4 tangs in my 400 g tank.
Here are the pictures of this fish (from Fishbase.org) http://www.fishbase.org/Photos/Thumbnai ... fm?ID=4742
I don't think it's the adult size.. part of it might be that the mimics aren't imported in vast numbers, so alot of shops don't even get them. Can't like it if you don't see it around.
I have a mimic tang (the pyroferus) that I have had since it was a juvenile. I love this fish!!!!
This is a combed-tooth tang similar to the kole, so does a bang-up job eating algae and diatoms.
Mine is in an intermediate phase right now between juvenile and adult colors. He is a beautiful dark dusky black mixed with yellow. Also has stunning eyes!
Why aren't more imported? Probably because they almost always come in as juveniles, and there are only a few months a year when they settle out of the plankton at this size.
Scott Michaels book says they grow to 9.8 inches. Thats no bigger then the ever popular purple tang or hippo? I agree that its probably because you just don't see them at the LFS