Can't be from a enemy that drops the Legendary like Hunter Hellquist with the Bee.
Mine was from a lvl 8 Pup Skag. Of all the enemies you fight, a lvl 8 Loaded Maggie just bursts out of this Pup Skag. My lvl 8 Maggie certainly made the 1st 15 or so levels much much easier, as if they weren't easy enough! Lol
I also found a lvl 48 Whisky Tango Foxtrot from a Rapid Stalker but after seeing how easy those damn shield booster killed me more than saving me I sold it haha.