I think when people indicate "Yellowtail Butterfly", they are referring to the Pearlscale - Chaetodon xanthurus.
I'm not sure if you have a FO
or a reef tank - I wouldn't keep this fish with corals -- many Butterflies eat coral polyps as part of their natural diet.
I'd feed it a wide range of foods, insuring that it gets a good selection of algae, as well as meaty (brine, mysis, etc.) foods. There are also some good sponge-based foods available that it will appreciate.
Here's a good article on Butterflys:
>>Good Butterflyfishes of the Genus Chaetodon<<