You're asking two different things. The diagram shows the different parts of the tenderloin from which the filet mignon comes.
The butt is the large end of the tenderloin.
A tenderloin can be peeled or unpeeled. Peeling involves the removal of external muscle and extraneous matter.
Costco around here sells the whole tenderloin (as pictured above) or a package with two butt ends. They also sell steaks cut from the tenderloin. I see them labeled as tenderloin steaks rather than filet mignon.
Taste is pretty much the same along the whole tenderloin.