The BUTT portion is more of a Club or Rib-Eye steak or a Prime Rib...
The butt portion has more fat and marbeling...
For filet mignon, what you want is the TOP-LOIN or short loin...
The short loin cuts also include Porterhouse & T-Bone steaks...

"[color:"blue"]The Short Loin Section
Most tender and most expensive beef cut. Can be prepared without the use of moist heat or long cooking periods. Cuts from the short loin section will do well when sauteed, pan-fried, broiled, pan-broiled or grilled.

Porterhouse Steak – very famous steak from the short loin’s rear end. It consists of both the strip steak and tenderloin. It is often served as filet mignon
T-bone Steak – cut from the short loin’s middle section. It’s quite the same with porterhouse steak and is usually pan-fried or grilled
Tenderloin – often regarded as the most tender beef cut. It can be cut into single filet steaks (filet mignon) or as a whole meat strip. The meat goes well with sauces, meaning to say the meat doesn’t overpower the sauce’s flavor[/color]"...
http://www.filet-mignon.org/meat-cuts/