I have worked at a LFS
that brought in a 18 inch long horn cowfish. They are ok for aquariums if you have several parameters.
1.) Carbon at all times
2.) Protein skimming
3.) Enough room for it to grow
and 4.) No other fish that it can pick at.
With all that said, I would be cautious with it in a reef. The little ones are sometimes ok, but in general, any one as it gets bigger will cause problems.