His name is probably Benico or Benedict or something srs like that, and Dean, being Dean, gives him a nickname in an unconscious attempt to make him less intimidating (that's my fanwank on why he nicknamed Cas, cause he was still pretty scared of him when he started calling him that).

Or Benny is a code name, the way they called Dean 'Guy' or something like that in the casting sides for 4x01 and 4x03, in an attempt to keep everyone from being spoiled about Dean's return or something.

Personally, I like Benny. It kind of reminds me 'The Vampire Jerry' in Fright Night. Goobery names for terrifying monsters amuse me.