There seems to be a unknown version of Bluto shown in two of TheMugenSpectator's videos
TMS Mugen Battle #177 - Maria and Broccalon vs Bluto and Voltorb - YouTube
TMS Mugen Battle #170 - Bluto and Popeye vs Peter and Homer - YouTube
I assume this is the same Bluto shown in this screenshot is also shown in both of TMS's videos so it doesn't seem it was a private character. This character could simply be lost in time and It will never resurface, however someone else that is active could happen to have this character.
EDIT: According to my research, the author of this Bluto seems to be Mario11766. Mario11766 is known for making sprite swaps of KFM, just like this Bluto.